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last four condensers can be connected as required to the earth or 
to the pole P of the Rumkorff coil to which the outer plates 
of N, and Ny are always connected. When the key V is shut the 
condensers are earthed together. From M a wire proceeds into 
the neighbouring room, and can be connected by the key 7 to the 
experimental condenser Z; the outer plate of which is always con- 
nected to the coil. 
Although only one adjustable condenser is required by the theory 
of the method, the second Ns is used partly for convenience, partly 
for the calibration of the former ') as described below. 
4. Calibration of the adjustable condensers. 
The adjustable condenser was, as mentioned above, an instrument 
constructed after Nernst’s design. The alteration in the capacity 
would be proportional to the movement of the glass plate if the 
apparatus were theoretically exact. This is naturally not the case, 
so the condenser must first be calibrated. The method employed is 
also due to Nernst. The calibration condenser is put in parallel 
with the adjustable condenser as described above, and the displacement 
determined which is required to re-establish equilibrium. 
When this is done the calibration condenser is again cut off, and 
equilibrium re-obtained with the help of the other adjustable con- 
denser. The former condenser is again put in parallel with the calibra- 
tion instrument, the necessary displacement measured and the process 
repeated until the glass plate is exhausted. In this way one finds 
the different positions of the glass plate which correspond to equal 
differences of capacity. The application of this method of calibration 
depends upon the supposition that the capacity of the non-inductive 
parts of the calibration condenser is negligible, as well as that of 
the wire which puts it in parallel with the adjustable condenser, 
The first condition is almost absolutely fulfilled by the condenser 
described above (see fig. 2). The use of the wire can moreover 
be avoided as shown by Nernst’). The condenser is insulated by 
an ebonite plate and is so arranged that the 2 mm. wire D, which 
projects above the cover of the condenser, stands at the same 
height as the wires of the adjustable condenser (Fig. 3). The cali- 
bration condenser can now be switched on and off by shifting it 
for about 2 mm. in a horizontal direction, so that the wire D is 
brought into contact with the lead. The respective positions of the 
1) See Nernst |, c. 
ARE 
