Ques. Describe one form of potentiometer.
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.
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.
Fig. 615.--To measure a pressure greater than 1.6 volts with Leeds and Northrup potentiometer by using a volt box or multiplier. To measure high voltages it is necessary to connect the voltage across high resistance and to measure on the potentiometer a definite fraction of the total drop. In the figure, AB is a high resistance of which CB is .1th, DB .01th, and EB .001th of the total resistance. The potentiometer reading is accordingly multiplied by 10, 100, or 1,000, depending upon whether the switch M is set on C, D, or E. Resistance boxes for this purpose are called volt boxes, and are constructed to multiply the potentiometer readings by 10, 100, and 1,000. In using them, it is only necessary to connect the unknown E.M.F. at the posts so marked, and the potentiometer to the posts marked P. The potentiometer reading is taken as above and multiplied by a factor depending upon the position of the switch M, which factor is indicated upon the box. It is essential in making these connections that the polarity be carefully observed.
How to Use a Potentiometer.--All connections must be made as indicated by the stamping on the instrument. Particular attention must be given to the polarity of the standard cell, of the battery, and of the voltage, the corresponding + and - signs being marked. If used with a wall galvanometer having a telescope and scale, it will be found convenient to place the potentiometer so that the telescope is directly over the glass index of the extended wire, thus permitting the observer to read the galvanometer deflections and potentiometer settings without changing his position.