140 Professors Ayrton and Perry on the Measurement 
The resistance, therefore, as measured by the galvanometer, 
does not increase as rapidly as the distance separating the 
plates, while that as measured by the electrometer is fairly in 
proportion to the distance. The explanation of the former is 
probably due to the fact that, since the electromotive force 
employed in all these four sets of experiments was constant, a 
greater current flowed when the plates were nearer than when 
they were far apart, hence that the resistance due to the layer 
of gas was greater when the plates were near than when they 
were far. 
And this leads to a simple method of accurately measuring 
the resistance of liquids by using a galvanometer. The 
method, which was independently arrived at by one of our 
assistants (Mr. Mather), is now employed in our laboratory, 
and is so simple that we feel it can hardly be novel. It is as 
follows : — In a long vertical glass tube containing the liquid 
there are two metallic disks, not necessarily of platinum, and 
of about the same diameter as the tube. One of these can 
slide up and down the tube, so as to be able to be set at any 
fixed distance from the other. The disks are first put tolerably 
far apart, and a certain convenient current made to flow, 
which is measured on a galvanometer in the circuit. The 
plates are now made to approach and the current kept exactly 
the same by the insertion of an external resistance; whence it 
follows that the resistance of the column of liquid which has 
been subtracted from that originally separating the plates is 
equal exactly to the external resistance necessary to be inserted 
to keep the current constant. 
February 28, 1878. 
The next set of experiments was made to determine the 
alteration in resistance of a long trough of water when the 
distance between the centres of the platinum plates was kept 
constant at 90 centimetres, and the positions of the platinum 
plates varied as shown in the figures. 
Galvanometer-Constant. — 4 Menotti's cells with an E.M.F. 
3'8 volts gave a deflection of 618 when a resistance of 10,000 
ohms was in circuit and the galvanometer shunted with the 
99 l, shunt. 
The 4 Menotti's cells were employed and the galvano- 
meter shunted with the ^~ shunt, and the reading- were 
in each case taken one minute after the application of the 
battery. 
