Introduction. — The ordinary galvanometric methods of com-
paring E.M.F.'s cannot be employed to obtain accurate results
when the cells to be compared polarize at all rapidly. In such
cases some ?iull method, such as Clark's, is preferable. The
following one (sometimes known as Law's method) can be con-
veniently employed when a condenser and ballistic galvanometer
are available. The condenser should have as small a capacity as
possible, but one that will give a satisfactory deflection with the
most sensitive arrangement of the galvanometer available. This
arises from the fact that if the capacity was larger, the larger
current necessary to fully charge the condenser might result in an
appreciable polarization. If one E.M.F., say E^, is much larger
than the other, it may be necessary to shunt the galvanometer G
with a shunt S when this larger E.M.F.
is being used {vide p. 113), so as to keep
the respective throws much about the
same in magnitude.
Apparatus. — Ballistic galvanometer,
G (p. 285) ; condenser, C; Pohl's com-
mutator, P (p. 330) ; batteries of E.M.F.'s,
El and Eo, to be compared ; key, K.
Observations. — (i) Connect up as
indicated in Fig. 49, and adjust spot of
light to zero.
(2) With K open, free the condenser
from any residual charge by short-circuiting its terminals; then
turn P over to the weakest E.M.F., Ej, for a few seconds.
(3) Turn P so as to flpe?i the Imttciy circuit, and quickly press K,




Fig. 49.
Practical Electrical Testing-



99



the spot of light being absolutely at rest. Note the first throw
dy of the spot.
(4) Repeat (2) and (3) two or three times, and take the mean.
