ON THE ELECTRICAL PROPERTIES OF PURE SULPHUR. 
77 
thousandth of the same, may be included in the galvanometer circuit in series with a 
megohm. A reversing key for the galvanometer and for the cell is provided. The 
resistances are adjusted till the first deflection of the galvanometer needle on reversal 
is, as nearly as may be, the same as the deflection when the sulphur is in circuit. The 
keys are arranged in such a way that the change from the sulphur circuit to the 
megohm circuit can be made almost instantaneously. In this way a direct value can 
be assigned to the current causing any deflection of the galvanometer needle, without 
relying on any particular galvanometer law ; both circuits are practically inductionless. 
It remains to measure the voltage of the storage cells. In our earlier experiments 
the cells were permanently arranged so as to be tested in sections against a set of 50 
Clark cells, or a smaller number by the method of differences employed by Lord 
Rayleigh in his experiments on the absolute electromotive force of the Clark cell (‘Phil. 
Trans.,’ 1885). The resistance of sulphur, however, depends so much on previous history, 
voltage, &c., that a refinement of this kind was soon seen to be unnecessary, and in 
ail the later experiments the voltage was obtained simply by placing the cells in series 
with a megohm and low resistance galvanometer, and observing the double steady deflec¬ 
tion. This was compared with the double steady deflection produced by the 100 volts of 
the electric light circuit, derived from 31 plate E.P.S. accumulators, and measured by a 
Fleming and Gimingham voltmeter, which had been calibrated against Clark cells, but 
which was found to require no correction at or near 100 volts. The proportionality of 
the readings of this galvanometer to the currents passing, within the limits employed, 
was established byaspecial seriesof experiments. The galvanometer was a low resistance 
reflecting galvanometer, very strongly controlled, and was read with a lamp and scale. 
It very soon became obvious—especially when the sulphur was heated—that some 
check must be devised, capable of detecting any breaking away of the film from the 
electrodes, owing to unequal expansion, &c. An arrangement of resistances for com¬ 
paring capacities by de Sauty’s method (see Glazebrook, ‘ B.A. Reports,’ Leeds, 
1890, p. 102, or ‘ Electrician,’ October 3, 1890) was accordingly set up in a permanent 
manner, and provided with keys, &c., so that the capacity of the sulphur cell could 
be obtained within a few minutes of the determination of its resistance. A certain 
'2 microfarad division of a condenser by Elliott Bros, was used as the standard, and 
this was afterwards compared with a condenser by Muirhead, whose corrections were 
obtained and furnished by Dr. A. Muirhead. The method used by Dr. Muirhead is 
described by Glazebrook ( loc . cit.). The uncertainties still remaining in the value 
of the condenser capacity are probably much too small to require further discussion 
for the following reasons : When we used the capacity test merely to check the 
adherence of the film, the absolute value of the standard did not matter, and when 
we used it to determine the capacity with a view to obtaining the specific inductive 
capacity of the sulphur, the small thickness of the film (about '25 millim.) prevented 
its actual thickness being measured with sufficient accuracy to make it worth while 
to endeavour to go behind Dr. Muirhead’s corrections. It is of course, easy to 
