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portional to the square of the current transmitted, it is obviously an advantage to make the transformation just noted.
The Construction of Induction Coils, although varying in details, follows the same general design. There is an iron core composed of soft iron wires, preferably No. 26 or 28, annealed to eliminate hard spots, and bound together in a round bundle. Enough wires are used to make a bundle 1 to inch in diameter, and their length is from 4 to 6 · inches. These are packed into a fiber tube upon which is wound the primary coil. This coil generally has not less than 300 turns and is wound in two layers. The size of the primary wire depends upon the kind of batteries employed and varies considerably. No. 22 or 24 B. & S. gage would be considered an average size. The number of turns in the secondary winding is usually from 8 to 10 times the number in the primary winding, and the size of the secondary wire is such that the necessary number of turns are obtained in 5 layers or less. No. 32 to 36 B. & S. gage is generally considered advisable. Single silk-cover. ed copper wire is mostly used for both the primary and secondary coils, and several thicknesses of paraffined paper are provided for insulation between these coils.
Fig. 12 shows a Monarch induction coil for ordinary local battery work. It is wound to a resistance of 1.7 ohms in the primary and 175 ohms in the secondary. The terminals are brought
out in heavy wire and are connected to binding screws on the fiber blocks P and S, the former of which contains the primary terminals and the latter the secondary terminals. The coil itself is covered with bookbinders' cloth as a protection from moisture.
The Battery.—Telephone work for the purposes considered in this book requires the use of primary batteries; in other words, batteries in which zinc is consumed by chemical action to generate
electricity. A battery consists of two or more cells connected, each of which comprises, besides the zinc or positive plate, another plate electronegative to the zinc, which may be either carbon or copper and which is called the negative plate. These two plates are placed in a glass vessel or jar containing an acid solution. The acid, attacking one of the plates more than the other, produces a difference of potential usually from 1 to 2 volts between them. Battery cells may be conveniently divided into two kinds: open-circuit cells and closed-circuit cells. The former are employed
where there is only an occasional use for the transmitters, and the latter where the transmitters are in almost constant use. All primary cells when working on circuits of such low resistance as those in which transmitters are used suffer a decrease in their electromotive force or voltage, and an increase in their internal resistance. This is due to the formation of small bubbles of hydrogen gas on the negative plate of the cell, which diminish the effective surface of the plate and set up an opposing electromotive force. It is, therefore, necessary to surround the negative plate with a depolarizer, the duty of the latter being to generate oxygen gas for combining with the hydrogen gas and thus setting it free in the solution. In open-circuit cells the depolarizer acts slowly and chiefly when the cell is not in use; in closed-circuit cells the depolarizer acts continuously.
The chief forms of open circuit cells are the Leclanché cell and the so-called dry cell. The chief forms of closed-circuit cells used in telephone work are the Fuller cell, the gravity cell, and the Edison cell. For furnishing current in the transmitter circuit it has become standard practice to use two Fuller cells in series, or their equivalent in the other forms of batteries mentioned. The cells are connected in series by joining with a copper wire the positive plate of one cell to the negative plate of the other, the remaining plates of the two cells serving at the terminals of the battery. When thus connected the total voltage is the sum of the voltages of the two cells, and the total resistance of the battery is the sum of the internal resistances of the two cells. The best results are usually obtained by having both cells of the same form and size, and both must be of the same
kind; that is, either open-circuit cells or closedcircuit cells.
The Leclanche Cell, Fig. 13, comprises a negative plate of carbon c surrounded by the depolarizer, which consists of a mixture of crushed manganesedioxide and crushed carbon in a porous cup d.
The positive plate is the zinc rod z, and this, together with the porous cup, is placed in a solution of sal-ammoniac and water in the glass jar m. The sal-ammoniac solution is best made by dissolving three parts of sal-ammoniac in ten parts
of clean soft water. This solution passes through the porous cup and moistens its contents. The binding posts or screws on c and z form the terminals of the cell. The resistance of this cell is usually less than 1 ohm, and the electromotive force is about 1.5 volts.
A modified form of the Leclanché cell is shown in Fig. 14. The depolarizer here
consists of two blocks, Fig. 14.-A Modified Form of e and s, of mangathe Leclanché Cell
nese dioxide and carbon clamped around the negative carbon plate c by means of rubber straps aa. The elimination of the porous cup considerably decreases the internal resistance of the cell. The binding screws on the carbon plate c and zinc rod z are the terminals. Sal-ammoniac solution is used as in