Page:Electrical Engineering Volume 1.djvu/288

1720 may be drawn up until the zinc is entirely out of the liquid; it is held in any position by the thumb-screw T. On the top of the brass rod is a binding-post B$1$, the other terminal of the cell being the binding-post B, which is connected to the two carbon plates.

The electrolyte is composed of 3 parts of potassium bichromate, dissolved in 18 parts of water, to which is added 4 parts of sulphuric acid.

The E. M. F. of such a cell is 1.93 to 2 volts.

At ordinary temperatures, variations in the proportion of bichromate in the solution, within moderate limits, do not vary the E. M. F. or the internal resistance very much. Variations in temperature vary the internal resistance, but not the E. M. F., the internal resistance decreasing as the temperature increases. With the above proportion of sulphuric acid and bichromate in the solution, the sulphuric acid is first exhausted. Theoretically, for an equal life of both substances in the electrolyte, the correct proportions should be

which proportion is often used. In fact, however, it is more necessary to keep up the strength of the depolarizer, that is, the bichromate, so the first given proportion will really give better results.

2625. A great variety of batteries of this type has been made, especially abroad, where they are called Poggendorf's cell; they do not differ in principle or material from the Grenet cell, but in mechanical details are more suited to general work. They are usually built with several cells, the various elements being connected in series to give an E. M. F. of 6 to 10 or more volts. All the elements are simultaneously raised out of or lowered into the liquid by a lever or windlass arrangement, as shown in Fig. 1039, which represents a battery of five cells all alike. The elements are of zinc and carbon, there being three plates of zinc, Z, and four of carbon, C, in each cell. The plates are