[CARR] AN EXTENSION OF PASCHEN’S LAW 165 
entirely confined to the space occupied by ebonite, so that in this 
way it was rendered impossible for a discharge to occur through the 
gas in any but a uniform field. The thickness of the ebonite ring, 
which could be made accurate to 4/;9), mm., determined the distance 
between the electrodes and consequently the length of the discharge. 
The length of the discharge could be varied at will, therefore, by 
inserting rings of different thicknesses. 
The gas was admitted and removed from the chamber by glass 
tubes sealed into the ebonite plugs, and these tubes were connected 
with the air space by two very fine channels leading through the 
ebonite ring. 
Before closing the discharge tube, which was made air-tight with 
ordinary commercial soft wax, the inner surface of the ebonite ring 
was carefully rubbed with glass paper to remove any conducting 
material from its surface. 
The potential differences used in these experiments were obtained 
from a series of small storage cells, similar to those used in the 
Reichsanstalt, Berlin. As these cells have a large capacity their 
voltage remained constant over long intervals of time, and as a con- 
sequence it was possible to make the readings with the greatest accu- 
racy. The potential differences were measured by a Weston voltmeter 
which was carefully calibrated by means of a potentiometer furnished 
with a standard Weston cadmium element. 
Throughout the investigation the discharge chamber was con- 
nected in series with a drying tube containing phosphoric pentoxide, 
a glass reservoir about two litres in volume, a McLeod pressure gauge 
giving readings accurate to */1 9), of a mm., and a mercury pump of 
small capacity. By using this reservoir and the pump of small capa- 
city it was possible to diminish the pressure in the discharge tube 
by such exceedingly small amounts that it was easy to obtain a series 
of discharge potentials over the whole range of pressures investigated 
without the necessity of admitting fresh gas to the chamber. 
In making measurements one terminal of the battery was joined 
to earth and the other terminal was connected through a resistance 
of xylol to one of the electrodes of the discharge tube. The other 
electrode was permanently joined to one pair of quadrants of a quad- 
rant electrometer, the second pair of which was kept to earth. In 
determining the potential difference necessary to produce discharge 
at a given pressure, the electrometer electrode was first earthed, a 
given potential applied to the battery electrode and the earth connec- 
tion of the electrometer electrode then removed. 
If, after waiting some minutes, no discharge passed, the operation 
was repeated with a slightly higher potential applied to the battery 
