Ans. As shown in fig. 614, it consists of a fine German silver wire about 3 feet long stretched between the binding posts A, B, which are attached to a wooden base carrying a scale divided into 1,000 equal parts. There are three circuits, the terminal A being included in each, one including the battery, and the other two the galvanometer. A three point switch connects the galvanometer in series with the standard cell SC, or the cell to be tested C, the circuits being completed by leads terminating in the sliding contacts M and S. [Illustration: Fig. 614.--Diagram of potentiometer showing method of measuring the voltage of a cell. The potentiometer is simply a high resistance wire of uniform diameter stretched between two binding posts, A and B, in such a way that contact can be made at its ends and along its length. Necessary circuits are plainly shown in the figure; SC, is a _standard cell_ and C, the cell to be tested. M, and S are sliding contacts, connecting with the "slide wire."] Ques. Describe the method of measuring the voltage of a cell with a potentiometer. Ans. Fig. 614 shows a method of comparing a pressure with that of a standard cell and is applicable whether the pressure of the cell to be tested be greater or less than that of the standard cell. In making the test the switch F is first closed, then the other switch is moved to D, and M adjusted till galvanometer shows no deflection; similarly, the switch is moved to G, and S adjusted till galvanometer shows no deflection. Then, C:SC = AS:AM. from which C = SC × AS ÷ AM. EXAMPLE.--Let 1.016 volts be the known voltage of the standard cell SC, and the scale reading of AS be 657, and of AM, 225 as in the figure, then C = (1.016 × 657) / 225 = 2.966 volts The arrangement may, however, be made direct reading, that is, the slide wire may have a scale of volts instead of lengths or resistances, as follows: Suppose the standard cell to have a pressure of 1.434 volts, the sliding contact M is placed at the reading 1.434, and the adjustable resistance varied till the galvanometer shows no current. This means that the pressure between A and M is 1.434, and consequently the pressures all along the slide can be read off the scale _in volts_. Hence, when S has been adjusted to balance, the pressure of C is read off the scale in volts.