Electrical standards

(a) Weston cell
This cell is used as a voltage standard, since its e.m.f. can be found very accurately. The positive electrode is mercury and the negative electrode an amalgam of cadmium in mercury, the electrolyte being cadmium sulphate solution. At temperatures between 0 ^oC and 40 ^oC the e.m.f is 1.0186 V
If currents of more than 10 mA are drawn from the cell it will depolarise, and so these cells must always be operated with a protective series resistor and never used as a source of current.

(b) Current balance
This instrument balances the gravitational attraction on a small rider against the electromagnetic force provided by a known length of wire carrying a current in a magnetic field. Sensitive current balances are accurate to five parts in a million; simpler current balances can be made, however, to check the calibration of ammeters