Part 1

The Thompson-Houston System of Electric Lighting.

Thesis submitted for the degree of Bachelor of Science in Mechanical Engineering,

to the Faculty of Purdue University

June 1887

“In its power to assume always that form of energy which happens to be the most useful lies the great importance of electricity.” This importance has been brought to the notice of the public by means of the many recent exhibitions. Public interest has been roused and there is everywhere a desire for information and a guide through this far reaching field for discovery and invention. And, although there are many works treating on electricity and electric light, people specially want a short and concise though thorough description of the various schemes by which electric light is produced. In this thesis the object is to give a brief treatise on one of the many schemes of producing light by electric currents viz—The Thomson-Houston System.

In pursuing the subject of electricity, the first thing noticed is the analogy and difference between the dynamo and its older and more powerful rival the steam engine. The resemblances are, First as in the development of the steam engine, but few of the improvements and inventions in electrical machines were made by mathematical leaders. Watt ran across the idea of the seperate condenser while repairing the Newcomen model and applied the expansion of steam to the steam engine by a mechanical accident rather than by his own ingenuity, and so we find the first designers of the dynamo were mechanics rather than philosophers. Secondly the tendency to disregard old methods and instruments because of new discoveries and inventions has, as in the steam engine, hindered the advancement in electrical science. As an example it has become customary to regard frictional and statical electric machines, for practical purposes, as obsolete, but recent discoveries seem to hint that they may yet be utilized. Lately Prof. Dodge has shown that dust and vapor whirling in the air may be settled by a discharge of electricity consisting of a continuous series of electric sparks. This has been utilized to clear the atmosphere in lead smelting works from the fumes of volatized lead and with its application comes the invention of Wimhurst which produces with a minimum of mechanical labor a continuous series of electric sparks and works admirably.

The differences between the engine’s and dynamo’s developement are: _First_ the marvelously rapid developement of the dynamo as compared with that of the steam engine. Since 1867 when the term “dynamo electric machinery” even to scientific men had but little signification, the dynamo has been brought to a very high degree of perfection. _Secondly_, the development of the dynamo has reached a much higher degree of perfection than that of the steam engine. Among the best steam engines twenty per cent effeciency is considered as very good while a good dynamo gives out in the form of electricity, ninety per cent of the mechanical energy put in it. But the class of people who improved and made the steam engine what it is were as well educated in one sense as were the men who brought out the dynamo. While it is true that in Watt’s time the knowledge concerning steam was very meagre, yet the practical men who _made_ the dynamo, did it by themselves as nearly all the teachers of electricity knew nothing except what may be called electrical tricks. As has been said[1] “The teachers and writers of textbooks, practically did not know that there was anything in common between the electricity from a rubbed glass machine and voltaic electricity, or to be brief, that there was a science of electricity as distinguished from mere natural history.” In fact as late as 1870 there were really no textbooks on electricity. Even now electrical knowledge is so meagre as to warrant the same writer’s expression, “We can not imagine a mechanical engineer mistaking a few inches for a few miles or a grocer compounding an ounce of sugar with a carload, but this gives too truthful an idea of the vagueness that still exists.”

In the distant future, electricity will be used for electric lighting only as subordinate to other uses to which it may be applied such as heating houses, taking place of stoves for cooking, being used as a substitute for the steam engine. In fact the motor is rapidly becoming of as much practical use as the electric light. The principle of the motor is just this; a certain amount of mechanical energy say thirty four horsepower per minute into the form of electric currents, which by the way gives enough current to run 45, 2000 candle power lamps, send the current and distance through suitable conductors and attach them to similar dynamo or dynamos but in such a manner that the current in the second set of dynamos flows in the reverse direction to that of the first; when, the armature of the second dynamo or dynamos will revolve and at the pulley or pulleys of the dynamos, aside from friction, will be given out 95% of the thirty-four horsepower, the loss being due to the resistance of the conductors. Now in practice a motor is placed on the arc light circuit the same as a lamp, for energy less than twelve horsepower. It does not affect the lights and is a clean, neat way of obtaining energy.

But however true the foregoing may be, the greatest present use of electricity is to start and maintain light. There are several so-called systems, embracing dynamos, lamps, regulators, etc, from which I select the Thomson-Houston as the one for the purpose of describing for several reasons, _first_, it is at least as good as the average system of which there is a mushroom growth; _second_, valuble information was kindly offered by the parent Company; _third_, a good plant is near to which free acess was given, and _fourth_, we have at the Mechanical Hall of this University, a dynamo, loaned by the parent Company, which affords information without any inconvenience. As each part of the system comes up to be described a little of its history will be given. As the first part of a system necessary to be produced is the current generator we will first describe

_The Thomson-Houston Dynamo._

In considering the current generator the first thing to be decided upon is the definition of the term dynamo. The following is thought to be a correct definition,—A dynamo or dynamo electric machine is a machine which is used to convert energy in the form of mechanical motion into energy of electric currents, or _vica-versa_. Those used to generate currents of electricity are called dynamos, those used to generate mechanical motion are known as motors.

In attempting to make clear the theory of the dynamo, we will recall some simple experiments. In Fig. 1, send a current around B from right to left. Now A being free to move vertically either up or down, connect its binding posts to a _galvanometer_ (that is, an instrument used to tell the direction of a current and also used to test the _relative_ strength of two or more currents) and move A up suddenly when a current will be generated in A whose direction will be the same as that of the current in B. Now this current is not created energy, because in lifting[2] the coil A, work is expending against the attraction between the coils, as between two currents flowing in the same direction there is an attraction. If we pursue this experiment in its various forms we will find the following statement known as Lentz law is true, viz: “If the relative positions of two conductors A and B be changed of which B is traversed by a current, a current is induced in A in such a direction that by its electro dynamic action on the current in B it would have imparted to the conductor a motion of the contrary kind to that by which the inducing action was produced.”

The theory of this law is that around every wire carrying a current there is a magnetic whirl (Fig. 3). Now if the conducting wire be passed through a hole in a horizontal plate of glass and iron filings be sifted upon the latter they will arrange themselves, as shown in Fig. 2., along lines, radial in this case, known as lines of force, which arranging is due to the magnetic attraction of the current in the wire upon the iron filings. Now in B. Fig. 1, every portion of the wire has just such a whirl and just such lines of force, or magnetic field, and when A is moved each part of the wire of A cuts one or more lines of force of the many magnetic fields making up the magnetic field of the entire coil B. Now when the wire of coil A cuts magnetic field of B a current is generated in A acording to the following statement known as Faraday’s Law; “When a conductor in a field of force moves in any way so as to cut the lines of force there is an electromotive force produced in the conductor in such a direction that supposing a figure swimming in the conductor to turn to look along the positive direction of the lines of force (in Fig. 1, toward axis of B), and the conductor be moved to his right, he will be swimming with the current so induced.” Hence in Fig. 1, the current generated in it will be from left to right.

Practically Faraday’s principle means just this: by moving a wire across a space where there are magnetic lines, the motion of the wire as it cuts the magnetic lines sets up around the cutting wire a magnetic whirl or in other words sets up a current in that wire.

The foregoing laws are the “principles of the dynamo,” yet after their deduction, the progress of the evolution of the dynamo was slow and attended by many dificulties. Between 1860 and 1870 however, a working knowledge of these laws became the property of thousands of mechanics, and by comparing the number of inventions before and after that date (1860) the present generous growth of systems, dynamos and lamps, prove that inventions were almost in proportion to the number of people who had any electrical knowledge. In 1866 Wilde produced a toy magneto-electric machine for giving shocks, in which he used excited electromagnets. In the same years Varley and others produced a machine which excited its own field magnets the type of all machines used in practice. With this principle of Varley’s and Pacinnotti’s ring, Gramme produced in 1871 his since famous continuous current generator, one of which the second dynamo electric machine ever brought to this country can now be seen at the engine house at Purdue University. In 1877 Silas Brush brought out his famous dynamo and it may be interesting to know that he designed and had one made without experimenting in the least. In the following year a patent was issued to Messrs. Elihu Thomson and Edwin J. Houston, Professors of electricity in Philadelphia on the present though much improved _Thomson Houston Dynamo._

To go back to Lentz and Faraday’s laws and carefully consider them we can but assent to S. P. Thompson’s “fifteen propositions on the dynamo” which are:—1. A part of the energy of an electric current exists in the form of a magnetic whirl surrounding the wire.

2. Currents may be generated in a wire by setting up these whirls.

3. We can set up these whirls by increasing or decreasing the relative distance between magnets and wires.

4. To set up and maintain these whirls consumes power.

5. To induce currents in a conductor there must be motion between them so as to alter the number of lines of force (Fig. 4 to 7).

6. Increase in the number of lines of force in the circuit produces a current of the opposite sense to decrease (Fig. 7).

7. Approach induces electromotive force in the opposite direction to that induced by retreat.

8. The stronger the magnetic field the stronger the current.

9. The more rapid the motion the stronger the current.

10. The greater the length of the conductor which cuts lines of force the stronger the current.

11. The shorter the conductor not so employed the stronger the current.

12. Approach being a finite process the approaching and receeding must give alternating directions to the current.

13. By the use of a commutator all the currents can be turned in the same direction.

14. In a steady circuit it makes no difference what kind of magnets are used to procure the requisite magnetic field whether permanent or electromagnets.

15. Hence the current of the generator may be used to excite the magnetism of field magnets.

Now the Thomson-Houston dynamo comes under that class of dynamos in which there is a rotation of coils in a uniform field of force, such rotation (Fig. 6.) being affected round an axis in the plane of the coil. Of course this dynamo is made like all others of its class to have _first_, as powerful field-magnets as possible, _second_, the armature or rotating coil has as great a lenght of wire in it as possible the wire being thick to offer little resistance and _third_, built to stand high rotative speed.

The simple theoretical dynamo is shown at Fig. 8, consisting of a single rectangular loop of wire rotating in the magnetic field formed by large magnets, and in order to take the current so generated from the loop so as to give a continuous current, we use a two part commutator (Fig. 9) consisting of a metal tube split in two and mounted on wood, each half connected to one end of the loop. The current is taken off by brushes which lead to the main circuit. But manifestly this dynamo would give no appreciable current becase it has a very small length of wire on the armature, so a great number of loops were used which at present constitute the so-called _drum armature_.

We may rotate the loops of wire in Fig. 8, on one of its sides as an axis or even push it farther from the center of revolution than that. To do this, wrap the wire around a ring and connect both ends to a two part commutator (Fig. 10). If instead of the ring in Fig. 10, being solid it be a number of coils of wire and if instead of there being one coil around the ring there be thirty we will have Pacinnotti’s ring before spoken of. If we used four to ten coils or “bobbins” of large size which is shown diagramatically at Fig. 11, we would have the Brush dynamo.

So with exceptions we may say that there are practically two types of dynamos as regards armatures, the _ring_ type as Brush, Pacinnotti’s Gramme, and the _drum_ armature (page 20).

The Thomson-Houston dynamo is like the rest of that dynamo, unique. To quote S. P. Thompson; “The Thomson-Houston spherical armature is unique among armatures, its cup shaped field magnets are unique among field magnets, its three part commutator is unique commutators.”

An armature of a dynamo is the rotating coil or coils which generates currents of electricity by moving in a magnetic field of force. It is the most important part of a dynamo as it is literally the current generator. So we first consider the Thomson-Houston

_Armature_

It is spheroidal in shape as is noted for the fact of its very seldom _burning out_, i. e., the electricity heating the wires of the armature to such an extent as to destroy the insulation or fuse the wires, either rendering the armature useless. It is made by keying two dish-shaped iron disks _SS_ Fig. 12, to the shaft x and putting ribs _dd_ about ten in number in the twenty-five light machine, and over the whole putting varnished paper. Then at stated intervals, pegs JJJ are driven into suitable holes in the disks and ribs to help in winding wire on the shell. Next three insulated wires of equal length are joined together at _h_ Fig. 13, and the three wires are then wound over the shell in the following peculiar manner: one half of No. 1 is wound so as to form a zone of a sphere of which the shaft is in the same plane as the center circumference of the zone. The armature is then turned on the shaft as an axis 120° and one half of No. 2 is wound in the same manner as the first half of No. 1. The armature is moved 120° more and all of No. 3 is wound. The armature is then turned back, 120° on the shaft as an axis and the remainder of No. 2 is wound. Lastly the armature is turned back 120° more and the rest of No. 1 is wound. They are bound by wires _gg_ Fig. 13 to hold them when rotating. The object of this rather complicated winding is to get the three coils equi distant from the shaft in order that each coil will generate practically the same current. Now as will be seen the overlapping wires will form a nearly spherical armature. The armature is mounted on the shaft _x_ as an axis which extends far enough out from its bearings to put a pulley on the end _H_ and a commutator on the other end to the three parts of which are fastened the three wires marked one, two, three, Fig. 13.

It has been urged that the repairs of this armature will be larger than on any other armature. If there should be a “burnout” it would necessitate the taking apart of the dynamo and sending the armature to the factory to be rewound. But it never burns out except through positive carelessness and it will be found that the repairs on this armature is less than on the armatures of its several powerful rivals taken separately even though they be of simpler construction.

When the Thomson-Houston armature is rotated between the cup-shaped fieldmagnets alternate currents are generated in each coil in turn and now the next point to be considered is the

_Commutator_

which incites the alternate currents so formed into one continuous current. The commutator as before stated is fastened on the shaft at the end one, two, three Fig. 13. It consists of three copper plates in the form of a cylinder each segment _A´A´A´_ covering 115° of the dotted circle Fig. 14. They are screwed to rod _CCC_ and _DDD_ which are insulated by wood and gutta-percha plates _EE_ from the iron mounting _E´´_ which is in turnescrewed to shaft by set screws shown. The wires one, two, three, have respectively red, white and blue insulation and are put in binding posts _DDD_ marked one, two, three at the factory and if not so placed may work badly. The current enters _D_ goes to _B´B´_ which there have direct contact with _A_.

Now in a three-part commutator the spark occurring as the segments pass under the brushes would very quickly destroy the surface and interfere with the currents in the coil. This difficulty is overcome by blowing out the spark by an air blast given at just the right place and time. The manner in which the blast is delivered is as follows: the segments of the commutator

are separated by gaps of about 5° and in front of each of the leading brushes there projects a nozzle, Fig. 15, which discharges an air blast alternately three times in each revolution. The blast itself is supplied by an ingenious piece of mechanism known as the

_Thomson Air Blast_.

It consists of an elliptical box II whose sides have perforations II where air can enter while inside of this rotates a steel disk keyed to the armature shaft and having radial slots in which slide three wings RRR of ebonite which as they fly around drives air into the holes JJ leading to the nozzles Fig. 15. The result is that, since the spark is done away with, oil can be supplied to the commutator in limited quantities but still amply sufficient to reduce the wear on the commutator to such an extent that the life of a segment is greatly increased. The air blast is fastened to the dynamo frame just behind the commutator and can be see in Fig. 23.

_The Field-Magnets_,

as may be seen from Fig. 17, consists of two flanged iron tubes _AA_ whose end consists of a convex segment of a sphere accurately turned to recieve the armature. Coils of wire _CC_ which are in the outside circuit and through which the entire current flows are wound upon the tubes. After the armature is placed between them the two tubes are bolted together by heavy wrought iron bars _BB_ and the whole carried on the frame work _PN_ shown also at _PN_ Fig. 23. Now a little magnetism only remains in the wrought iron bars and iron frame works when the armature first revolves, but the current even though slight, going through the coils makes an electromagnet out of each tube and heavily magnetizes the wrought iron bars and in two or three seconds after the armature first rotates it is entirely surrounded by a heavy magnetic field. One of the good points of these field magnets is that but very little magnetism is lost as compared with most other dynamos and since it takes power to maintain a heavy magnetic field, this dynamo is in this respect very economical.

_The Thomson Regulating Gear_

Later on we will show that pushing the brushes together or pulling them apart alter the strength of the current, but for the present just accept the fact and we will show how the brushes are varied. It is accomplished by the mechanism shown in Fig. 18. The brushes are fixed to the levers YY and Y_{2}Y_{2} united by the lever _l_. The automatic movement is obtained by the electromagnet _R_ while a dashpot _J_ prevents too sudden motion. Suppose the brushes to be in the position shown when the current would get too strong owing to lights being cut out. The electromagnet R getting stronger would raise _A_ and reduce the current taken off until current came to normal. If, instead, some lamps were thrown in the current would become weak and the electromagnet _R_ would become weak, drop A which would increase current and this will continue till current reaches normal.

The foregoing regulating gear is used on small dynamos and old style large ones. On the large new style dynamo a more delicate regulating gear is used, the current which operates it being shown at Fig. 19. Normally the electromagnet _R_ is short circuited by the wire _r_ and only acts when this circuit is broken. At some point in the main circuit is a _wall controller_ or _controller magnet_ shown in Fig. 19, at ST, consisting of two electro magnet, Their yoke supported by a spring and the yoke operating the contact lever S. If the current becomes too strong the controller magnet circuit is broken and all the current of the main circuit goes through the electromagnet _R_ which by its sudden increase of strength quickly raises _A_ and thus alters the brushes. This only exists for a moment until the yoke of the controller magets fall because of their decrease of magnets strength, when current again flows through wire _r_ because when yoke drops contact is made. This decreases the strength of electro magnet _R_ thus dropping _A_ and increasing current. Hence _S_ will again raise and break contact and _R_ again rais _A_. This is continually repeated.

_The Brushes_,

of which there are four in use on all machines, are made of a broad strip of springy copper having six slits two thirds the distance up, and thus touching at several points. They are held by clamps shown at Fig. 23 which also shows the brushes. The brushes are held to the commutator by their own springiness and the variation of position due to strength of current. The brushes are set by a gauge sent with each dynamo which shows length from the end of brush to the holder. The holders are set at the correct angle by a gauge of brass of the shape of a right angled triangle the short side having a wide flange curved to fit the commutator for which it is sent, while the second side as regards length must fit to the holder when swung to it on the commutator as an axis.

After describing the details of the dynamo, we will at once proceed to find how the