( 214 ) 
2 measurable capacity. Naturally this is not easily attained, but the 
respective corrections may be determined fairly simply as explained 
below. 
2. Description of the separate apparatus. 
The electrometer was a THoMsoN’s in its original form. When 
this is set up in the neighbourhood of the working pumps of 
the eryogenic laboratory it must be little sensitive to vibration. 
For this purpose I obtained the damping by air (after TöPLER), 
instead of by sulphuric acid, and replaced the bifilar suspension of 
two cocoon fibres by a platinum wire about 70 em. long and 30 u 
in diameter which can carry a much greater weight. 
After these changes had been made the vibrations were less than 
0.1 mm. on a seale at 3.5 m. distance, even when the pumps of 
the cryogenic laboratory were working at a distance of 10 m., while 
with the original arrangement it was quite impossible to make obser- 
vations under the same conditions. 
The induction coil was worked by an alternating current making 
200 vibrations per second, the spark distance at the ends of the 
secondary being about 0.05 mm. }). 
For the condenser an apparatus was used which was constructed ° 
according to NERNST*) viz. two metal plates between which a glass 
plate can be displaced. Theoretically the alteration in the capacity 
is proportional to the position of the glass plate. In practice however 
the condenser must first be calibrated. This can be done with the 
help of a specially constructed condenser such as Nernst’s Trough 
condenser *). The one used and shown in Fig. 2 only differs from this 
in that the ebonite cover has been replaced by 
a metal one; and the metal tube, to which the 
other plate is fastened, by a stiff wire D of 2 mm. 
diam. The latter is insulated from the metal cover 
by a small thickness of ebonite. In a condenser 
constructed thus the non-inductive capacity is 
exceedingly small. 
In the construction of the experimental con- 
Fig. 2 denser there are two points to be primarily con- 
sidered; as large a capacity as possible must be introduced into a 
somewhat limited space, and the non-inductive capacity which is 
connected to the electrometer must be as small as possible. These 
1) Preliminary experiments showed that the passage of sparks of several mille- 
meters long did not explode liquid nitrous oxide though this is endothermic. 
2) Nernst, Zeitschrift fiir physik. Chemie XIV. 4. 
*) Nernst, loc. cit. 
7 
