550 Proceedings of the Royal Society 
of resistance during the earliest stages of polarisation! Accordingly, 
his criticism is, that contact cannot be so quickly made and broken, 
i.e., the time of passage of current cannot be made so short, as 
to warrant our neglecting the effect of polarisation; and he cites as 
proofs of the fact the experience of Kohlrausch and Nippoldt,* and 
the experiments of Edlund.f “If,” he writes, “the authors were 
acquainted with Edlund’s experiments, they would know that 
platinum electrodes, between which the current from three Daniell’s 
cells has passed in dilute sulphuric acid during only — th of a 
second, have already acquired a polarisation, whose electromotive 
force is equal to that of 0*57 of a Daniell’s cell.” Very interesting, 
hut unfortunately not to the point ! Any one who understood the 
method of our paper would know that Edlund’s experiments were 
quite irrelevant, simply because our observation was equally accu- 
rate whether the current flowed during yi-g, 1 or 100 seconds. In 
what respect then did the method differ from Professor Beetz’ idea 
of it? In one respect, viz., that while he supposed us to have 
used a heavy mirror galvanometer, we used Sir William Thomson’s 
“ Dead Beat ” galvanometer J — and we distinctly stated that such 
an one was necessary for the use of our method. § This galvano- 
meter has four peculiarities: — (1.) The mirror is exceedingly light; 
(2.) On the back of it there are four very small magnets; || (3.) The 
mirror cell is just large enough to admit of deflection; (4.) The front 
and back of the cell act as stops. In virtue of the former two 
peculiarities the mirror moves almost instantaneously in obedience 
to even a very weak current; and in virtue of the latter two, there 
is almost no oscillation. The effect of these properties is seen by 
comparing an observation made by means of the ordinary galvano- 
meter with one made by means of the “ Dead Beat.” Suppose the 
ordinary galvanometer to be used in a Wheatstone’s Bridge, one of 
the arms of which is a tube containing an electrolyte with platinum 
electrodes, while the other three are known resistances, and are 
* Pogg. Ann. cxxxviii. p. 282, 1869. 
t Ibid, lxxxv. p. 209, 1852. 
J See Fleming Jenkins’ “ Electricity and Magnetism,” p. 198. 
$ P. 58 of our paper. 
|| The mirror and magnets of our galvanometer weighed together only 
about -08 grm. 
