Base, Key, and Condenser.

Fig. 46.

The base for a Ruhmkorff coil generally resembles an oblong shallow box. The coil is mounted on the lid, and the condenser inside the box, the connections being made on the lower side of the lid. It is preferable, except for appearance’ sake, to make all connections outside the box, but this is left to the worker’s choice.

Fig. 47.

The Condenser is made up of 70 sheets of tinfoil each about 4 inches by 7-1/2, and 80 sheets of clean white paper 5 by 8 inches placed alternately, and saturated with paraffin wax. The tinfoil sheets are laid so that about 1/2 inch projects out of the paper sheets at each end, the alternate sheets coming out at the same end, and the projecting pieces being bent together gives the effect of a pair of tinfoil sheets insulated from each other, aggregating the sum of all the small ones.

Fig. 48.

The coil can now be attached to the base by means of screws passing through the lid into the coil ends. If a vibrating contact breaker be desired, reference to Fig. 44 will show method of connection. Fig. 47 shows details of a contact breaker of similar design. R is hammer head of soft iron, S a spring about thickness of clock spring and 3/8 inch wide or more. B is contact point, both spring and adjustment screw A being fitted with platinum contacts. C is a check nut, to hold A from turning. I is an adjustment to tighten or loosen spring S, by means of a lug which it carries on its shaft. It is well insulated from pillar carrying A, by the hard rubber bushing, I.

The condenser is laid in the box under the coil and attached as in Fig. 44; that is, one set of sheets to the contact pillar, and the other set to the adjustment screw.

For gas-lighting work, it is generally preferable to use a contact or strap key (Fig. 48), instead of a vibrator. The key can be mounted on coil base, in which case the condenser will be attached in same manner as for the vibrator.

CHAPTER VI.
How to Select Batteries for Gas Lighting.

Before entering into a description of the various batteries used in electric gas lighting, it will be well to briefly consider a few simple electrical rules bearing upon the subject.

A current of electricity has electromotive force, or difference of potential figured in volts, and current figured in amperes.

For example we will use the water analogy (Fig. 49). Two tanks, A B, on the same level, are connected by a pipe C.

Supposing tank A be filled with water and the pipe, C, to be opened; the water will flow along C into B until the level in each tank is equal. So long as there is a difference of level, there will be a pressure in C, owing to the water behind it.

Replacing the tanks A and B by unequally electrified bodies, and the pipe C by a conductor of electricity, the flow of water is represented by the tendency of the electrified bodies to equalize themselves by a flow of current along the conductor, C.

To sum up: The difference of level is now difference of potential, the pounds pressure along the pipe being expressed as electromotive force in volts.

Fig. 49.

The quantity of water flowing along the pipe is measured, as electricity, in amperes. As the quantity of water passing in a given time is regulated by the size of the pipe and its own pressure, so the quantity of electricity is also regulated. A conductor of electricity offers resistance to the flow of current according to its sectional area and the material of which it is composed, this resistance being expressed in ohms. The greater the voltage and lower the resistance, the more current. This law, and its kindred applications, are expressed as follows:

C = E/R.

C is current in amperes, E electromotive force in volts, and R resistance in ohms.

Thus a wire with a resistance of 50 ohms would pass 2 amperes with an electromotive force of 100 volts. To find resistance when other two factors are known, the formula is

R = E/C.

In selecting a battery for work, regard must be made to the current required, and its period of flow. For energizing a gas lighting primary coil, the current must be large, but is only required occasionally, the battery standing idle for long periods. In this case the class called open circuit cells are preferable, as they contain no strong acids and do not deteriorate to any extent when not in use. Of such class is the Leclanche-Samson, Monarch, carbon cylinder, and most so-called dry cells. As the resistance in a conductor affects the current flow, so it does in a battery cell; the internal resistance of a battery is determined by its size, proximity of the elements, etc. Cells with small zincs and porous cups are of high internal resistance, those with large sheet zincs and big carbon surfaces, of low internal resistance. As the primary coil used in gas lighting is never much over one ohm, a cell of low internal resistance should be selected. But as the wires leading to the burners must be taken into account, a number of cells should be used to produce enough electromotive force to overcome the added resistance. Now battery cells can be arranged in a variety of ways—in series for higher electromotive force, and in multiple—for greater current.

Fig. 50.

Fig. 50 represents the series arrangement; here the zinc of one cell is connected to the carbon of the next; this adds the electromotive forces together and thus gives greater ability to overcome resistance, but it also adds together the resistance of each cell. Thus 4 cells, each 2 volts and of one-half ohm internal resistance, would, in series, have an E. M. F. of 8 volts and an internal resistance of 2 ohms, current 4 amperes. Fig. 51 shows four cells in multiple, the zinc of each cell and the carbons of each cell are connected. Here the result would be but 2 volts, but the internal resistance would be only one-quarter, viz: one-eighth of an ohm, current 16 amperes.

Fig. 51.

The readiest rule for connecting a battery is to arrange it according to the resistance of the line or outside wiring. So as we will have to use house-wiring far exceeding in length that on the coil, and probably of less diameter. Therefore the series arrangement will be the one to use, and not less than four cells of a low-resistance battery.