114 
MESSRS. J. HOPKINSON AND E. WILSON ON THE 
one aniotlier, and from the flask by means of sealing wax and glass tubes, dip into 
the sulphuric acid forming the inner coating of the jar, and similarly, electrodes h, d 
dipping into the outer acid make connection with the outer coating. The acid inside 
and out was made to wet the flask up to a level higher than the acid would reach at 
the highest temperatures. 
The four electrodes, «, h, c, d, are connected respectively by thin copper wires, 
with four mercury cups 1, 2, 3, 4, cut in a block of parafiin, and, by means of a 
]‘eversing switch a, h and c, d. can be connected respectively to mercury cups 5, 6. 
Cups 5, 6 are connected respectively to 7, 8 by thin wires, which can in turn be 
connected with or disconnected from the source of charge 9, 11. 
The steady potential difference of about 1,500 volts is obtained from a Siemens 
alternator A, in series with a revolving contact maker B fixed to the alternator shaft 
and making contact once per complete period, there being six periods per revolution. 
The contact-maker is set to make contact when the potential difference is a maximum. 
A condenser C, and a Kelvin vertical electrostatic voltmeter V, are placed in parallel 
between the connecting wires leading to mercury cups 9, 11, 
The galvanometer G has a resistance of 8,000 ohms and is inclosed in an iron box 
which acts as a magnetic shield. The box is supplied with a small window for the 
ray of light to pass through it from an incandescent lamp to the mirror from which 
it is reflected back through the window to a scale at a distance of 12 feet from the 
mirror. The divisions on this scale are v^tli of an inch apart, and an average 
sensibility for this instrument is '3 X 10"''^ ampere per division of the scale. The 
galvanometer is supplied with a shunt S, and has its terminals connected to mercury 
cups 13, 14 on the pciraffin block. These mercury cups are connected to cups 10, 12 
respectively, which can at Avill be connected to 7, 8, by one motion of the glass 
distance-piece g forming part of the reversing switch Avhich places 9, 11, or 10, 12, in 
contact Avith 7, 8. A switch is so arranged that 13, 14 can be connected at Avill, 
that is, the galvanometer is short circuited. 
The process of charging, discharging, and observing, is as folloAA’s:—Near the 
observer is a clock beating seconds Avhich can be distinctly heard by the observer. 
Initially, the cups 9, 11, are disconnected from 7, 8 ; but 5, 1, and 6, 2, are connected. 
At the given moment the reversing SAvitch is put over connecting 7, 9, and 11, 8 ; 
the jar is then being charged tJu'ough electrodes a, h. This goes on for the desired 
time, during which charging volts and zero of the instrument are noted. At the end 
of the time recpiired for charge, the main reversing SAvitch is put over connecting 
7, 10, and 8, 12 ; next the subsidiary switch is put oAmr connecting 3 to 5 and 4 to 6, 
and on opening the short-circuiting SAvitch, the spot of light is deflected and alloAved 
to take up its natural state of movement determined by residual charge, readings 
being taken at stated epochs after discharge is started. This whole operation, 
including an adjustment of the shunt Avhen necessary, Avas so speedily accomplished 
that reliable readings could be taken 5 seconds after discharge is started. By using 
