In many cases it is desired to operate a Ruhmkorff coil from an electric-light main direct. This can readily be done if the circuit be of the constant potential class—that is, one constructed to furnish current for incandescent lamps in multiple. With the direct current, such as the Edison, all that is necessary is either to interpose a rheostat, as in Fig. 64, or to use the lamps, as in Fig. 65. The manner of connecting up is the same as if the storage cell B be replaced by the coil. Using the formula C = E/R, for example, if the circuit be at 110 volts and the coil require 10 amperes, a resistance of 11 ohms will be required. Or using the lamps in the diagram, Fig. 65, about 20 lamps are to be put in circuit. If the current be an alternating one, the contact-breaker will have to be screwed down or short circuited.
The "U. S." Storage Cell.
This cell is of the lead-zinc type, being the practical form of the Reynier cell. It is to be recommended for working Ruhmkorff coils, its output weight for weight being far in excess of the lead-lead types. This cell is readily portable and easy of operation, the zinc electrode being the only one needing renewal, and that at very infrequent intervals.
The lead electrode consists of plates of peroxide clamped together, and presents quite a large surface. The zinc in most types is of the circular sheet form, and encloses the lead block, being kept amalgamated by mercury lying in the bottom of the cell. The E. M. F. on open circuit is about 2.5 volts, which is higher than any lead-lead combination. On closed-circuit work this drops to from 2.35 volts downwards. During action, when a large amount of current is being drawn from the cell, a white sulphate appears, but this disappears upon the cell being recharged or even left to rest. Bubbles of gas, which sometimes form under the peroxide block, should be removed by gently tilting the cell or hitting the table or shelf upon which it stands a smart blow. The large type No. 3 is suitable for X-ray work, and a still larger cell is made, which is preferable for heavy or continuous discharges of current.
Harrison Cell.
The No. 1 cell recently put upon the market has given excellent results for open circuit work. It consists of a negative element with lead peroxide as a depolarizer. The positive element is self-amalgamating zinc, which is free from local action. The electrolyte is dilute pure sulphuric acid. The potential is high, being 2.5 volts, and the internal resistance is 0.14 ohm. This cell belongs to a group which is midway between primary and storage, or secondary cells. Its construction is similar to the lead-zinc secondary cell, in place of which it may be used, it being easy to recharge an exhausted cell by passing a weak current through it in reverse direction, thus recharging the peroxide of lead grid and renewing the zinc and electrolyte.
The large size, or type No. 3, which the manufacturers are producing, differs from the No. 1 cell in that it has a larger negative element, or grid, and has two zincs, instead of one; consequently, it has a lower internal resistance—0.07 ohm—and a higher discharge rate with a capacity of 150 ampere hours. The potential is 2.5 volts. It is suitable for coil work or for sparking gas engines, and for ease of manipulation and convenience is to be highly recommended.
Fig. 67.