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KF5JRV > TECH 01.12.16 15:24l 8 Lines 1146 Bytes #999 (0) @ WW
BID : 6291_KF5JRV
Read: GUEST
Subj: Standard Cell
Path: IW8PGT<CX2SA<N0KFQ<KF5JRV
Sent: 161201/1315Z 6291@KF5JRV.#NWAR.AR.USA.NA BPQ6.0.13
In calibrations with the potentiometer it is necessary to have a "normal" or "standard" cell of known and constant e.m.f. The two cells used universally for this purpose are the cells devised by Latimer Clark and by Edward Weston. A form of the Clark cell is shown in Fig. 339. The positive pole is mercury (Hg), in contact with a paste of mercurous sulphate (Hg2SO4), and the negative pole is zinc in contact with a solution zinc sulphate. When this cell is made strictly according to the specifications fixed by the national physical laboratories, it has an e.m.f. of 1.434 volts at 15°C. and for a temperature t , an e.m.f. of [1.434 - 0.0012 (t - 15)] volts.
The Weston cell is exactly like the Clark cell except that the zinc is replaced by cadmium, and the zinc sulphate by cadmium sulphate. Its e.m.f. in the standard form is 1.0190 volts, and it has the great advantage of having practically no change of e.m.f., with temperatures. No appreciable current should be taken from a standard cell, as the accompanying chemical actions cause more or less permanent changes in the cell and its e.m.f.
73 Scott KF5JRV
KF5JRV.#NWAR.AR.USA.NA
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