(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.