Fig. 65.
Fig. 65 shows the employment of lamps instead of the rheostat. The lamps L L regulate the current flow by the manner in which the circuit is arranged. If only one lamp be turned on, the current necessary for only one lamp circulates through the battery. Each additional lighted lamp adds to the current by decreasing the resistance of the circuit. S is a switch which must always be left open when the dynamos are to be stopped.
Charging from Primary Battery.
In many instances an electric-light circuit is not available for charging purposes, in which event recourse must be made to a primary battery. The one most suited for the work is the modified Daniell, or copper and zinc combination in solutions of sulphate of copper (bluestone) and sulphate of zinc respectively.
There are many good forms of this cell on the market, chief of which are the simple gravity, the Gethins, and the Hussey, which have been previously described. An example will now be described of the operations necessary with the gravity cell, charging one 50-ampere hour storage cell. At least six cells of gravity will be required, as the voltage of each cell is never over 1 volt, and is dependent on the resistance in the external circuit falling as the resistance is lowered. Place the six clean glass jars on a firm foundation, where there is no liability of shaking and no dust likely to settle. Unfold the copper strips into the form of a star, bending the corners for half an inch so as to give an anchorage in the bluestone. Place them into the bottom of the jars and pour in water enough to cover them at least 3 inches below the surface. Now carefully drop in 4 pounds of clean bluestone, which will fill in the angles between the copper wings, at the same time holding the element down to the bottom of the jar. Hang the zincs from the top edge of the jar, and fill up with water to 1 inch from the top. The addition of 5 ounces of sulphate of zinc per cell will render the cells immediately available, and for the further hastening of the chemical action, the copper wire from each copper may be inserted in the binding post-hole of the zinc belonging to its own cell and screwed tight for a few hours; or the cells may be connected together in series, and the wire from the last copper be screwed to the zinc of the first, thus putting the whole series on short circuit. The only advantage of the first method being a saving of time when a number of cells is being set up. This saving of time is often of consequence, as the longer the newly set-up cell is on open circuit, the more copper will be deposited on the zinc, which is highly undesirable. This is shown by the blackening of the zinc as soon as it is put in the solution, which blackening it is hard to prevent entirely. When the cell is working satisfactorily it will show a clearly defined line between the colorless solution above and the deep blue solution beneath.
Gravity cells should never be moved. If no sulphate of zinc is available, half a teaspoonful of sulphuric acid may be poured in over the zinc, which will tend to form the sulphate of zinc. Without any of these helps the cell will take at least twenty-four hours on a short circuit before it will give its normal current. This current should be from 4∕10 to 5∕10 of an ampere. Five cells set up by the writer varied after the addition of the zinc sulphate from 200 milli-amperes (thousandths of an ampere) to 300 milli-amperes, although they were apparently all set up alike; but after twelve hours' short circuiting they all gave a fairly uniform current of from 470 to 500 milli-amperes.
