CHAPTER X.

BATTERIES FOR COILS.

In selecting a battery to operate the coil, one is needed which will supply a large steady current for a considerable period. Although the primary circuit is opened and closed rapidly, yet the class known as open circuit cells is not suitable, even though they have a low internal resistance, and thereby render a large current. Such cells are only suitable for the uses for which they are mostly designed, bell-ringing or annunciator work. There is one case, however, where an open circuit cell may be used with an induction coil, and that is in gas lighting as previously described; but here a dozen or so impulses of current are generally sufficient, followed by long periods of rest. For the latter work the cells in common use are the Samson, Champion, and Monarch, all of which are of low internal resistance and great recuperative power.

The reason that such cells will not work for long periods, is that they polarize. This latter action takes place in these open circuit cells, which are of the Leclanché type as follows: A positive plate of zinc is immersed in a solution of ammonium chloride (or salammoniac), and a negative plate of carbon and peroxide of manganese, contained either in a porous cup or compressed into a block also stands in the solution. Care is taken that these two plates do not touch each other. When the outside circuit is closed the zinc combines with the chlorine of the solution liberating free hydrogen and ammonia. The hydrogen appears at the negative plate, where it is acted upon by the oxygen of the peroxide of manganese to form water.

But when the circuit is of too low resistance, the oxidizing action of the peroxide of manganese is not rapid enough, and a film of hydrogen, which is a poor conductor, forms over the negative plate, increasing the internal resistance of the cell and setting up local action. In the best class of these open circuit cells, this hydrogen is absorbed after a rest, and the battery recuperates and is ready for work again. The circuit of the Ruhmkorff coil is low, and this polarization always occurs a few minutes after the contact-breaker is started.

In the class of closed circuit cells, chosen for the present purpose, the Grenet or bottle bichromate is one of the handiest for occasional use. A glass bottle-shaped jar, _J_, Fig. 59, is provided with a hard rubber cap, _G_, on which are mounted the binding posts _A B_. To the underside of this cap are attached two carbon plates _C C_, which reach nearly to the bottom of the jar, being connected together on the cap by a varnished copper strip, the latter being in turn connected to one binding post. Through the centre of the cap passes a brass rod, _R_, having attached to its lower end a piece of sheet zinc, _Z_, well amalgamated with mercury. This process of amalgamation consists in cleaning the zinc, then rubbing its surface with a rag dipped in dilute sulphuric acid, and pouring a few drops of mercury on the wet zinc. The mercury will spread readily over the zinc, provided it has been well cleaned, and if properly done should give the zinc plate a bright, shining appearance.

When the cell is not in use, the zinc is drawn up into the neck of the bottle and clamped by a set screw against the brass rod. A copper spring pressing on the rod serves to carry the current to the second binding post.

This cell originated in France, whence its name, but a cheaper form is now made in the United States known as the Novelty Grenet. The shape of the jar is somewhat different, and the carbon is moulded, whereas the French carbon is sawed from the carbon deposited in the gas retort; but the American form is practically of as great utility as the French, and the cost recommends it.

The bichromate solutions are affected by light, and deteriorate less it kept in stoneware jugs. The Grenet battery can very well be fitted into a neat wood case, which will serve the further purpose of preventing chance knocks from fracturing the glass jar.

Carbons which are used in batteries containing the foregoing solution should be well washed in warm water whenever the solution is changed, and especially when it is intended to put the battery out of active service. When the solution acquires a decidedly green hue it should be replaced with fresh. The electromotive force of this cell varies from 1.90 to 2 volts, and the amperage is dependent on the size of the plates, running from 5 amperes upward.

The glass jar is filled up to the commencement of the neck with a solution of bichromate of potash or sodium, called electropoion fluid, and prepared as follows: To 1 gallon of water add 1 pound of bichromate of sodium, mixing in a stoneware vessel. When dissolved add 3 pounds of sulphuric acid in a thin stream, stirring slowly. As the mixture heats on the introduction of the acid, care must be used to pour in the latter slowly. This solution should not be used until quite cold.

The sodium salt is preferable to the potassium, owing to its not forming the crystals of chrome alum, and also on account of its lower cost and greater solubility, the latter being four times greater than that of the potassium salt. The commercial acid used should contain at least 90 per cent pure acid and should be free from impurities. On filling the battery use utmost care not to splash the solution on any of the metal work, or it will cause corrosion. Although the salts in the solution will most likely make a stain, the corrosive action of the acid can be arrested if the solution be splashed on the clothes by the prompt application of ammonia solution.

The "Fuller" cell, Fig. 60, which is another type of the bichromate cell, is one from which a steady current can be obtained for a longer interval than from the Grenet, but the current is less. The electromotive force is the same, but the current is only 3 amperes, except in certain modifications.

In the porous cup is a cone-shaped zinc having a stout copper wire cast in. This wire is occasionally covered with rubber insulation, but, as a rule, is bare. The porous cup is of unglazed porcelain, thick, but very porous. This sets in the glass jar, a wooden cover fitting _loosely_ over the whole to exclude dust. Through this cover passes the wire leading from the zinc, and also the carbon plate carrying a machine screw and check nuts for connection. The cover is dipped in melted paraffin, as is also the upper end of the carbon and the rim of the glass jar. This is to prevent the creeping of the salts in the solutions and the corrosion of the brass work.

Into the porous cup is poured a solution composed of 18 parts by weight of common salt and 72 parts by weight of water. Electropoion fluid is held by the glass jar, the two solutions reaching a level of two thirds the height of the jar. One ounce of mercury is added to the porous cup solution to ensure the complete and continuous amalgamation of the zinc. The salt can be more readily dissolved in warm water, but _all_ solutions must be used _cold_. It is not always necessary to renew the solutions when the battery fails to give out its accustomed strength, but several ounces of water can be substituted for a similar amount of fluid in the porous cup. Stir the solution by moving the zinc up and down, and a temporary improvement will be noticed.

To obtain a greater current from this cell, use a larger zinc, such as a well-amalgamated zinc plate, and add a teaspoonful of sulphuric acid to clean water for the porous cup solution. Additional carbon plates connected together and placed round the porous cup will lower the resistance of the cell and increase the current, and also tend to keep down the polarization.

A new form of this battery was described by M. Morisot a short time ago.

The positive pole is of retort carbon in the outer cell in a depolarizing mixture made of 1 part sulphuric acid, 3 parts saturated solution bichromate of potash, crystals of the latter salt being suspended in the cell to keep up the saturation. A porous cup contains a solution of caustic soda. The zinc is in a second porous cup placed within the first, which holds a caustic soda solution of greater density. The electromotive force is 2½ volts when the cell is first placed in circuit, and will remain at 2.4 for some hours. The internal resistance is low, but varies with the thickness of the porous cups. This cell is not suitable for any but use for a few hours at one time.

The Dun cell has a negative electrode of a carbon porous cup filled with broken carbon. The zinc is in the form of a heavy ring, and hangs at the top of the solution in the outer jar. Permanganate of potash crystals are placed in the porous cup, and the entire cell filled with a solution of caustic potash 1 part to water 5 parts. The voltage is 1.8, and the internal resistance being low the resultant current is large.

A cell with an electrode of aluminum in a solution of caustic potash and carbon in strong nitric acid in porous cup is claimed to have an electromotive force of 2.8, but the nitric acid is not a desirable acid to handle.

Metallic magnesium in a salammoniac solution with a copper plate in a hydrochloric acid and sulphate of copper mixture is of high voltage, nearly 3 volts being obtained, and the current is large, but it is a new combination and has not as yet stood the test of time.

There are other formulæ for solutions to be used in Fuller or Grenet cells which may be useful to the experimenter. Trouvé's is as follows: Water, 36 parts; bichromate of potash, 3 parts; sulphuric acid, 15 parts, all by weight. Bottone's: Chromic acid, 6 parts; water, 20 parts; chlorate of potassium (increases electromotive force), ⅓ part; sulphuric acid, 3½ parts, all by weight. A convenient "red salt" or "electric sand": Sulphate of soda, 14 parts; sulphuric acid, 68 parts; bichromate of potash, 29 parts; soda dissolved in heated acid, and potash stirred in slowly. When cold can be broken up and prepared when required by dissolving in five times its weight of water.

The chromic acid used in Bottone's solution is very soluble in water, it being possible to dissolve five or six times the amount in the same quantity of water as of bichromate of potash. The simple solution of chromic acid is 1 pound to 1 pint of water, to which is added 6 ounces of sulphuric acid.

When it becomes necessary to cut zinc plates, it may be readily done by making a deep scratch on the surface, filling the scratch first with dilute sulphuric acid, and then with mercury. The mercury will quickly eat into the metal, and the plate may be easily broken across or cut with a saw. Zinc plates can be bent into shape by the application of heat. Hold the plate in front of a hot fire until it cannot be touched by the bare hand: it will be found that it has softened so that it can be bent around a suitable wooden form. As zinc plates are most attacked at the surface of the acid solution, it is advisable to coat the extreme upper portion of them with varnish or paraffin. Rolled zinc is always preferable to cast, especially so when immersed in acid solutions.

To avoid confusion, it may be stated here that it is the rule to speak of the zinc element as the positive plate and the negative electrode or pole, and the carbon _vice versa_. The portion of the element immersed in the solution is the plate, the part outside, the pole or electrode. In diagrams and also in formulæ positive is shown by a + (plus) sign and negative by a-(minus) sign.

The relation of cost of the materials most used is shown in the subjoined table, which cost, however, varies with the market:

Sulphuric acid, chemically pure 18 " " commercial 1.5 Muriatic " 1.12 Nitric " 3.5 Electropoion fluid 2 Bichromate of potash 10.5 " " soda 8.5 Caustic soda 9 Salammoniac 7 Chromic acid 19 Blue vitriol 4 Litharge 5.75 Mercury bisulphate 94 Paraffin 9 Beeswax 35 to 45 Shellac varnish 87 Tinfoil 35

GRAVITY BATTERY.

A cheap modification of the Daniell cell. A glass jar has at the bottom a copper plate consisting of 4 to 6 leaves of thin sheet copper, set on their edges in a starlike shape, a copper wire being attached to the copper rivet which holds the leaves together. A mass of crystals of sulphate of copper is filled in and laid on the top of the copper electrode an inch or so above its top. The negative plate is a variously shaped plate of cast zinc hung from the edge of the jar and reaching about 2 inches from the top into the fluid. Water is poured in until it covers the zinc, and the battery is complete. The sulphate of copper deposits its metallic copper on the copper leaves and liberates sulphuric acid, which rises and attacks the zinc, setting free sulphate of zinc. The sulphate of zinc solution being of greater density remains near the bottom, and the sulphate of zinc solution stays near the zinc. When the cell is left too long on an open circuit the two solutions tend to mix, and copper is deposited on the zinc. The sulphate of zinc finally saturates the top solution, which has to be partly drawn off and replaced by fresh water and crystals of sulphate of copper dropped into the jar to take the place of that which has been decomposed. Electromotive force 1 volt, current from 3∕10 to 5∕10 of an ampere. The practical working of this cell will be treated of later on in these pages.

The Gethins (Fig. 61) and the Hussey bluestone cells both have the zincs standing in porous cups (shown by dotted lines), which in turn are supported half-way down the jar, generally resting on the copper strip acting as a porous partition between the fluids. The zinc stands in a solution of zinc sulphate, or a weak sulphuric acid solution. The internal resistance is low, and the current large, being from 1 to 5 amperes. These cells are the ideal bluestone cells for charging storage batteries requiring very little attention. The special Gethins cell shown in the figure has the copper made with a collar, which encircles the porous cup, and thereby lowers the internal resistance of the battery. The voltage not being over 1 volt, however, renders these cells hardly suitable for direct connection. Five cells connected in multiple would give all of 10 amperes of current, and 1 volt, and a number of these multiple groups could be connected in series for a higher voltage.

GORDON BATTERY

is similar in operation to the Edison-Lalande, but differs in details of construction. The zinc is a heavy ring suspended outside, but not touching a perforated tin cylinder closed at the bottom, containing the oxide of copper in flakes. Its internal resistance is slightly higher than the Edison-Lalande cell, otherwise there is little choice. The 6 × 8 size is excellent for coil work, giving 250 actual ampere hours and remaining on open circuit for long periods without deterioration.

EDISON-LALANDE CELL.

This is a practical form of the old Lalande-Chaperon cell, and gives a steady, large current, being of low internal resistance, but is of low electromotive force, being less than .70 volt on closed circuit of medium resistance. Being of low internal resistance, however, its output is large—three cells of the type _S_; internal resistance, 0.025 ohm. Capacity, 300 ampere hours, will about equal one cell type E 5 of the Chloride Storage Battery. The elements of this cell consist of positive plates of amalgamated zinc, suspended on each side of negative plates of the black oxide of copper in an electrolyte solution of caustic potash. In action the decomposition of water forms an oxide of zinc from the positive element, which with the potash in combination leaves a soluble salt of zinc and potash. The hydrogen of the water acts on the oxide plates to form metallic copper, thus really reducing, instead of increasing, the internal resistance of the cell. A layer of heavy paraffin oil is poured on top of the solution to prevent the action of air.

NEW STANDARD,

or Roche dry cell. This cell possesses remarkable recuperative powers and low internal resistance. Made in many sizes, the best suited for medical coils is No. 2; dimensions, 5-7∕8 × 2-7∕16 inches. For heavier work the No. 5, 6 × 2-9∕16 inches, and known as the Navy Standard, is recommended. A convenient size for portable medical coils is No. 3, 3¾ × 1⅞ inches, taking up very little room, yet giving a large output. Two of these latter cells enclosed in the coil case will give with a suitably wound primary (No. 18 to 20 B & S) as strong a current as can be used in electrotherapy. For Ruhmkorff coils cells Nos. 6 and 7 (6 × 3 inches and 7 × 3 inches) furnish a most desirable battery for all work not needing the constant operation of the contact breaker, such as wireless telegraphy, gas-lighting, etc. They will do service on X-ray work, but the writer prefers a storage cell or the copper oxide types. The E. M. F. of the above cells is one and six-tenths volts, and current from 9 amperes to the No. 7 size, which gives 24 amperes on short circuit.

DRY-CELL CONSTRUCTION.

As a matter of practice, there is no really dry cell; all so-called cells contain liquid held in suspension, and their output is limited to the amount of fluid. One of this type can easily be made in the following manner: A containing jar is made up of first-quality sheet zinc, the edges being joined by a turned seam and then soldered, the bottom of zinc being also soldered in. In soldering here, as actually in all such operations, be _absolutely sure_ the edges of the metal are clean. The jar is partially filled with the following composition: Oxide of zinc, 1 part; sal ammoniac, 1 part; plaster of paris, 3 parts; chloride of zinc, 1 part; water, 2 parts, all by weight; or sal ammoniac, 1 part; chloride of calcium, 5 parts; calcined magnesia, 5 parts; water, 2 parts, or enough water to make a thin paste. A brass binding post is soldered to the zinc case and a carbon plate having a binding post is inserted in the centre of the cell, care being taken that it does not touch the zinc. A small disc of wood laid in the bottom of the cell will prevent contact at the bottom. Molten pitch or a composition of pitch and rosin in the proportion of 6 to 1 is poured on top, so as to seal the cell. As gas is generated in the cell, a safety valve should be provided, either a piece of porous cane or a short length of hard rubber tube, inside of which have been placed a few strands of woollen thread. This class of cell is so cheap and so many forms are available for choice that it is rarely desirable to make one's own. They will not do for steady current, but only for intermittent work. The large sizes being of low internal resistance, can be used for signalling in wireless telegraphy, where it is not possible to use wet (or free fluid) cells. The principal dry cells on the market are the Mesco, the O. K., the Nungesser, and the Samson semi-dry cell.