CHAPTER II.

CONTACT BREAKERS.

The simple form of contact breaker already described is useful up to a certain point, but it has disadvantages. Its rate of vibration is only variable through narrow limits, and it is not suitable for very heavy currents. But as it stands it has done long service, and will be used probably wherever the requirements from it are not exacting. The most desirable form of this simple spring break is shown in Fig. 18. _R_ is the soft iron armature; _S_, the spring; _C_, check-nut which holds the adjusting screw _A_ from becoming loose; _T_, a second adjusting screw used to tighten the spring and so raise its rate of vibration; _K_ is the base to which one wire of the coil is attached; _L_, base of adjusting device to which battery wire runs at _I_. Where tightening screw T passes through the pillar of the adjusting screw, the hole therein is bushed with rubber to prevent accidental contact. Both _A_ and _T_ are provided with insulating heads of rubber or ivory. At _B_ are the platinum contacts, which should be fully ⅛ inch in diameter.

One serious defect in the action of the simple spring vibrator (Fig. 19) is the tendency of the spring to vibrate, as it were, sinusoidally. This causes an irregularity in the rate of the vibrations, which affects the discharge of the coil very considerably. By far the better plan is to use a very short thick spring riveted to an arm carrying the armature at its end (Fig. 20). _R_ is the armature, _S_ the piece of spring, and _K_ the point of attachment to the base. The actual width of the portion of the spring which vibrates—the hinge portion, it might be called—should not be over ⅛ inch.

The rate of motion is high; but an erroneous notion has been taken of its performance by many persons in the knowledge of the writer. The rate of vibration is _not_ wholly dependent on the size, or, rather, smallness of its spring; the arm and armature considerably alter this, although they are not pliable, by reason of their mass and the momentum consequent on their mass.

A word here on the size of the armature. It should be somewhat larger than the face of the electro-magnet core, and should be thick—that is, in a circular form—say one half its diameter. Of course this does not apply to the steel lever armature before mentioned. It is impossible to lay down arbitrary rules where the conditions are not determined, but a very small amount of experimenting will demonstrate the correct lines on which to build.

When in action, all rapid rheotomes give out a definite musical note whereby the rate of vibration can be determined. Reference to any work on acoustics will show a table of the number of vibrations necessary to produce any stated musical note. The foregoing style of rheotome forms the basis of very nearly all those which are in use. The shorter and stouter a spring the more rapidly will it vibrate, and _vice-versa_. Carrying out this rule, we can manufacture an instrument which will give as high as 2500 vibrations per second (Fig. 21).

The armature _A_ is a piece of flat hard steel bar ¼ × ½ inch, held rigidly on the metal support _S_ and just clearing the upper surfaces of the magnet cores _C_. The adjusting screw _P_ should be provided with an arm, _B B_, whereby the rotation of it can be delicately varied. This screw must also be firmly held or the high speed of the armature will jar it loose. A check-nut on each side of the frame carrying it should be provided in every case. The necessary platinum contact can be hammered into a hole drilled before the armature is hardened. The proper place for this contact is about one fourth of the total length of the armature from its support, although in the simple contact breaker it can be placed at the distance of one third if desired. The reason is that the concussion of the adjusting screw dampens the free vibration, and the amplitude thereof is lessened in addition to the counter vibrations of the screw disturbing the regular vibrationary series.

Owing to the fact that the amplitude of the armature vibration is so small, a very delicate adjustment is necessary. The adjusting screw can be placed nearer the free end, but for the reasons given it is not to be desired. The metal bridge should be a solid casting, and the armature clamped by more than one screw.

The mercury vibrator, which is applied to almost every large coil, is as follows:

A pivoted arm carries on one end a soft iron armature, which is attracted by the coil core. The other end is provided with a platinum point adjustable by a set screw. This platinum point dips into a mercury cup—a glass cup containing mercury, with a thin layer of spirits of turpentine. The object of the spirits of turpentine, which is a non-conductor, is to help choke off the spark which would ensue whenever the platinum point was raised from the mercury.

A form of contact breaker which will admit of great variation of speed, and which is adapted to carry large currents, is the wheel-break, constructed in the following manner:

A brass or copper disk 3 inches or more in diameter and upward of ½ inch thick has its periphery divided by a number of saw cuts, which divisions are often filled in with plugs of hard rubber or fibre. This disk is mounted on a shaft, which latter is either the shaft of an electro-motor, or is provided with a pulley by which it can be rapidly rotated. A strip of spring copper on each side of the disk presses upon the toothed surface, one strip being connected to the coil and the other to the battery or other current source. It will now be seen that when the disk rotates the slits or pieces of hard rubber cause the break in the circuit through the brushes or copper strips, the rapidity of the breaks depending upon the rate of rotation of the disk, and the number of slits in the wheel.

The slits or rubber pieces should be one-half the width of the intervening brass, but must be at least one sixteenth of an inch in width, especially where a high voltage is used in the primary coil.

The shaft of the machine may serve as one point of connection in place of one of the copper brushes; but in this event either a wide journal must be used, or else some conducting substance, as plumbago, replace the lubricating oil in the bearings.

POLE CHANGING BREAKER.

Fig. 22 shows a diagram of a pole changing contact breaker which will allow of rapid alternations of current. It is operated by an electric motor by preference, although any motive power can be applied to it.

_W a W b_ are two brass wheels, the peripheries of which are broken by the insertion of insulating blocks _I I_, shown black in the sketch. _S S_ are the shafts on which the wheels are mounted, the two wheels being necessarily insulated from each other. 1, 2, 3, 4 are four brushes of copper pressing on the rim of the wheel and leading in the current from the battery _B_. The primary coil is attached to the brass body of the wheel or to the shafts. When the wheel is in the position shown, the coil and battery are on an open circuit; but on the wheel commencing to revolve, the brushes 1 and 2 bear on the brass, and the current flows from the positive pole of the battery to 2 through the wheel _W a_ to the coil _P_, up through wheel _W b_ and out at 1 back to the battery. The next position of the brushes 1 and 2 will be on the insulations, and 3 and 4 will come into action. Then the positive current will reach _W b_ by means of brush 3, and after traversing the primary coil and wheel _W a_, emerge at 4 to the battery, thus reversing the current through _P_ as many times as there are sets of segments, which latter can be multiplied according to requirements. The main point to be considered after that of good connections is that the brushes 1 and 3 and 2 and 4 do not at any time touch any part of the brass wheel at the same time, as this would short circuit the battery. This is avoided by making the insulating space longer than the brass surface, and adjusting the brushes as in the sketch, that each pair of them is a fraction further apart than the length of the brass tooth.

Accordingly, a wheel may be constructed with many segments and rotated at a high speed and rapid reversals of current produced, the uses of which are manifold.

As will be described in the notes on the Tesla effects, an electro-magnet, the poles of which are brought near the sparking point of the contact breaker, will help wipe out the spark, and so assist the suddenness of the break.

An extremely successful expedient in operating contact breakers is to employ a high-pressure air blast directed point blank against the contact point. The effect of this air blast when the contact is made is of course null, but on the platinum surfaces becoming separated, the high air pressure produced forms a path of extremely high resistance, and tends to blow off the spark as soon as it is generated. The stream of air should issue from an insulated nozzle of glass or rubber, and should not contain moisture.

WEHNELT INTERRUPTER.

One of the most important inventions in coil work is the electrolytic interrupter of Wehnelt. Briefly, the apparatus consists of a vessel containing a solution of acid, into which dip two electrodes connected in series with the source of power and the primary of the coil. Upon passing a current through the combination the fluid becomes agitated at the electrodes and a rapid make and break of the current ensues (Fig. 23).

It requires considerable electromotive force for operation, a minimum of 40 volts being desirable. Its rapidity of action varies up to and at times exceeding 4000 interruptions per second. A Wehnelt interrupter can be made as follows: Procure a glass jar _J_ holding about one quart or a little less, also a cover for same _C_, a piece of sheet lead _L_ large enough to fit loosely across the jar and yet not touch the bottom, eight inches of one-quarter-inch glass tube _M_, a few inches of No. 20 platinum wire _P_, and two ounces of mercury. Heat the end of the glass tube in a gas flame, and bend an inch or less at a right angle; at the same time seal in the platinum wire by means of a blowpipe, so that the tip just projects from the bent end of the tube. This sealing can be accomplished readily by one unused to working glass, but almost any philosophical instrument maker will have it done at small cost. Holes being bored through the cover, the lead plate and the glass tube are fitted in, the platinum point almost touching the lead. Adjustment is, however, easy, as the tube, being turned, will retract or advance the platinum point from or towards the lead electrode. Nearly fill the jar with a solution composed of one part sulphuric acid to eight parts water, and fill up the glass tube with mercury. The connections can then be made by means of a clamp on the lead and a wire dipping into the mercury. Connect the lead plate _L_ to one pole of the battery or source of energy, and the platinum-mercury electrode _F_ to one post of primary. The other side of battery and coil being closed, the apparatus will begin to work. No condenser is needed with this interrupter.

DESSAUER CONTACT BREAKER.

This is a modification of the spring hammer-head type, but has a platinum contact on both sides of the spring. It thus obtains double vibrations, but is liable to stick. The elasticity of the spring normally prevents the circuit remaining closed on the forward movement of the hammer head, but this combination requires attention.

STEEL RIBBON INTERRUPTER.

For light currents and rapid vibrations, such as are employed in electrotherapy, the steel ribbon interrupter is suitable. It consists of a steel ribbon _V_ one-half inch wide by six or eight inches long and the thickness of a stout visiting-card. Near the end is riveted a platinum contact. One end of the ribbon is held by a brass upright _R_, to which connection is made to circuit; the other end is riveted to a threaded rod, which passes through a brass pillar, and is held by a thumb-screw and check nut _S_. Turning the thumb-screw either way tightens or loosens the ribbon and so raises or lowers the rate of vibration (Fig. 24).

CONTACT BREAKERS IN VACUO.

Contact breakers in vacuo, as applied to Ruhmkorff coils, are by no means of recent date. Poggendorff made use of such prior to 1859, and noted the diminished sparking at the contact breaker and increased effect in the secondary circuit.

Mr. D. McFarlan Moore, whose experiments in vacuum tube lighting have proven so interesting, was granted patents upon various forms of contact breakers, in which the chief merit was that the contacts were broken in a vacuum. The sparking was almost eliminated, and the suddenness of the break of contact so accentuated as to materially improve the output of an induction coil. A perusal of his patents, copies of which may be procured through almost any bookseller, will prove profitable to the coil constructor.

QUEEN CONTACT BREAKER.

The most important advantage of this arrangement is the abrupt break, owing to a collar in the vibrator striking a movable contact while at full speed. Reference to Fig. 25 will show that the movable platinum contact is carried on a small vertical spring behind the vibrator spring, and projects through a collar on the vibrator spring. When the contact is made, the movement of the vibrator is not arrested, but continues at its full amplitude, thus allowing a long "make." The vibrator is kept moving at a constant amplitude by means of the small coil shown in the illustration, which is in shunt with the main circuit. In the old forms there has always been a liability of the platinum contacts sticking (or welding together). In the new form, as the break is made when the vibrator is in the middle of its swing, the sudden blow with the entire momentum of the iron hammer head is always sufficient to break the platinums apart. This form of contact breaker is very efficient on electric-light circuits, and operates with the utmost regularity.

THE QUEEN CONTACT BREAKER FOR LARGE COILS.

This is a device where the actual break is made in alcohol between large studs of platinum nearly one-quarter inch in diameter. The bottom contact can be raised or lowered by means of an adjusting screw. The top contact is secured into the bottom end of a rod passing down a guide tube into the alcohol to meet the lower contact. By means of an electric motor and a cam motion, the top contact and plunger are made to work up and down in the alcohol, thus making and breaking the current flow. One of the commendable features of this contact breaker is that the platinum studs are caused to revolve while in operation, thus presenting new faces to each other after each blow. The apparatus is not adapted for rapid action, but for the handling of heavy currents.

ADJUSTABLE CONTACT BREAKER FOR MEDICAL COILS.

An adjustable contact breaker for medical coils is shown in Fig. 26. _M M_ are the magnet coils, _A_ is the armature, carrying a platinum contact, which vibrates against the adjusting screw _P_. The armature is pivoted at _J_, but is held at a distance from the magnets by the springs _S S_. The other end of the armature carries a ball _B_, which can be slid up and down on the rod and set at any point by a set-screw. When the ball is at the end of the armature rod most remote from the magnets, the vibrations are slowest; when moved towards the magnets, the vibrations become more rapid. Adjustment of the two springs _S S_ at _R R_ enables the contact breaker to operate on varying current strength, and also tends to lessen the jerkiness of gravity contact breakers. A flat spring, however, can be substituted for the spiral springs, in which case the pivot would be dispensed with and the spring riveted, as in the hammer form of vibrator. The illustration shows this arranged for a wall board, but it can readily be adapted for table work.

ADJUSTABLE CONE VIBRATOR.

Fig. 27 shows a form of contact breaker much used in portable medical coils for slow speeds. It consists of a cone of iron _H_, mounted on the vibrator spring, and furnished with adjustable contact spring and screw _A_. Its amplitude of vibration is limited by the two pins mounted on the disc, between which the cone vibrates. The disc is turned by hand, thus moving the pins, and so varying the travel of the cone _H_ to and from the core _C_. It does not give good results from the fact that the rhythmical movements are disturbed every time the cone strikes against the pins, also at the contact spring striking the contact screw. As we showed before, a really satisfactory contact breaker should have a spring, which allows of no sinusoidal movement. Where a pivoted armature is governed by a spiral spring, the result is a series of steady, rhythmical shocks, provided the adjustments are satisfactory.

COIL HEAD CONTACT BREAKER.

Fig. 28 shows the details of a contact breaker to be attached to the coil head direct. It is often used on very small coils, which, together with a miniature dry cell, is slipped into a pocket case. An important detail in small coils is to use a contact breaker of sufficient size. Most of them are not large enough to stand ordinary usage, the adjusting screw is not of sufficient diameter and the thread soon strips. There is no reason why the adjusting screw, its platinum tip, and the pillar or lug which holds it should not be solidly built, it would certainly require less adjustment. Either single or double check-nuts can be fitted to the adjustment screws of nearly all the forms of contact breakers described.

CONTACTS.

It is absolutely essential that the _diameter_ of contacts for all contact breakers should be as large as possible and their faces filed truly parallel to enable them to easily carry all the current required. One of the main causes of failure of coil is burning of the platinum point and platinum burr, the current being then materially reduced. Large sparks at point of rupture are often indications that the condenser is not working properly—perhaps has broken down or is not large enough. The contacts will sometimes fuse together; at any rate, the excessive sparking is an evidence of waste as much as in a dynamo generator.

The adjustable method of arranging condensers (see Chapter IV.) is here of great value, but it is easy to attach more condenser sections to the contact screw pillar and vibrator pillar and notice result. In the construction of Ruhmkorff coils it is a good plan to make all connections possible on the coil base, instead of inside the condenser chamber. This is done either by means of rubber-covered wires or neat strips of brass, screwed down on the base from points of connection, and, of course, carefully bent over or well insulated from all other leads which they have to cross.

The best makers of induction coils construct their instruments so that they can be readily taken apart with as little detachment of connections as possible.