551 
of Edinburgh, Session 1874-75. 
arranged in such proportion that, if the tube were replaced by an 
equal metallic resistance, a very small deflection of the mirror, in 
a positive direction, would be obtained on closing the circuit. 
Then, during the time of contact, a large deflection is produced in 
a negative direction. The moment of inertia of the mirror is so 
great, that before the main current has moved it, at least perceptibly, 
in the positive direction, the polarisation current carries it off in 
the negative. If contact lasts -g^th of a second, the deflection is 
due to the sum of all the forces acting upon the mirror during that 
space of time. Against such a method Professor Beetz’ criticism 
is valid; because it is almost impossible to make and break contact 
in less than ^th of a second, and we certainly did not think that 
we had done so. But suppose that we use the “ Dead Beat ” gal- 
vanometer, the bridge being in the same condition. During 
contact the mirror makes two deflections,— the first, very small 
and in a positive direction, the second, much larger and in a 
negative direction, — the size of the second deflection depending, 
within limits, upon the length of time of contact, while both the 
occurrence and size of the first are entirely independent of it. The 
inertia of the mirror is so small, that the main current — lessened of 
course by the first traces of polarisation — produces its effect 
before polarisation has had time to gather its forces; and it is this 
first deflection, caused by the main current, which is observed, and 
which is reduced to just nothing by changing the relation of the 
arms, in order to determine the resistance of the tube. It is thus 
evident that the length of time of contact has no effect upon our 
result. So far as a single observation is concerned, it is quite the 
same whether it lasts a long or a short space of time. Edlund’s 
experiments and Kohlrausch and Nippoldt’s experience are thus 
alike worthless to a critic of our method, and Professor Beetz cites 
them simply because he was criticising a conception of his own. 
He was, perhaps, led astray by the importance we attached to 
making the time of contact as short as possible. But this precau- 
tion had reference to the next following observation. The shorter 
the contact, the less time required for depolarisation and the less 
change in the constitution of the liquid. The same remarks, which 
I have made to shew that we did not lean upon such a broken reed 
as the shortness of contact, make it evident also that we were right 
