Equivalents of Oxygen and Hydrogen. 617 
resistance-box giving tens, units, and tenths, and capable of 
standing the current used (0*6 ampere at the most). R is a 
fine adjustment, which though very simple is, so far as my 
experience goes, new, and proved extremely satisfactory. It 
consists of a glass tube (1 mm. bore, 25 cms. long) closed at 
the bottom ; a platinum wire is sealed through the bottom, 
the tube filled with mercury, and a steel wire inserted at 
the top. By pushing the wire down or pulling it up, the 
resistance of the circuit can be adjusted with ease and 
certainty to a thousandth of an ohm. 
By adjusting R and R/, the potential difference between 
p and q is kept constantly equal to one, two, or three 
cadmium cells C by means of the galvanometer G. The 
cells, which were made in the laboratory, are kept in a 
water-bath. 
G is a direct-reading galvanometer, by R. Paul of London, 
indicating nearly one scale-division per microampere. 
K' is an ordinary tapping key, kept down when desired by 
a weight. 
Conduct of an Experiment. 
The electrolyte having been filled in, current is started, 
with the tap L open. The current is then adjusted so that 
the cadmium cells are balanced, and is left flowing for a few 
minutes. The coulometer is usually covered with a wet rag 
to keep its temperature down. When the current is steady, 
all the temperatures are noted. Current is broken for fifteen 
seconds, during which time the tap L is closed, and a re- 
adjustment of the rheostat is made : this is necessary because 
the gas-bubbles in the coulometer increase its resistance ; 
hence, after the break of fifteen seconds the current would 
be stronger than before and out of balance. With practice, 
the rheostat can be adjusted so that the current is almost 
•exactly balanced immediately the circuit is remade. The 
plug of key K is then inserted at a precise moment by the 
chronometer, and attention devoted instantly to adjusting 
the rheostats, while an assistant turns the tap of the measure- 
tube to allow the mercury to flow out at such a rate as to 
keep the gauge I level. After the first minute one observer 
can attend to both matters, and it is easy to keep the mercury 
flowing so that the pressure of the gas in the measure-tube is 
never more than a centimetre or two of oil above or below the 
atmospheric pressure. Under favourable circumstances the 
electric current can be kept steady as exactly as the galvano- 
meter can be read, which is about 3QQQ part ; and of course, 
even when there are sensible fluctuations, they are sometimes 
