50 J. G. MACGREGOR ON THE 
great and a measurement made even a small fraction of a second after the cessation of the 
polarizing current is very much too small. But when the polarization is produced by the 
passage of the polarizing current during only a fraction of a second or a few seconds (in 
the method of measuring resistance referred to above, the current passes during only a small 
fraction of a second) the rate of dissipation is very small * and hence a measurement made 
shortly after the cessation of the polarizing current though too small will be but little too 
small. Of the two applicable methods the direct electrometric method was the one chosen 
and Thomson’s Quadrant Electrometer was the instrument employed. The differences of 
potential of the electrodes of the respective cells were measured alternately during a certain 
length of time beginning as soon as possible after the polarizing current had ceased, and 
the times of such measurements were noted. The times were noted by means of my watch, 
as I had no chronograph at command. It was therefore possible to draw for each cell a 
curve shewing in a graphic manner the rate at which polarization diminished with time. 
If the polarizations at the moment of cessation of the polarizing current were the same, 
these curves must coincide, as the conditions of dissipation are in that case identical. Ifthe 
curves are found to coincide, the rate of dissipation of polarization is the same for both cells 
after a certain time from the cessation of the polarizing current, and as the initial value of 
the polarization is one of the quantities on which its rate of dissipation depends, it is highly 
improbable that the initial values of the polarization can have been different. 

The arrangement of the apparatus is shewn in the diagram. A and B are the elec- 
‘trolytic cells of equal section and containing electrodes of platinum plates of equal area. 
E is the Electrometer; G, a Battery of one or more Grove’s cells. 6, d, e, f, g, h, k, l, m, n, 
0, and p represent mercury pools in blocks of paraffin and the lines from them to various 
parts of the cells, battery, and electrometer represent wires between these points. The 
pairs # and /, m and m, and o and p could be simultaneously joined by means of bent copper 
wires fixed to a stick of sealing wax. When this contact piece was in the mercury pools 
the current from G passed through both B and A in the same direction. If these pairs 


* See Maxwell, Electricity and Magnetism, Vol. I, p. 321. 
