406- 
ME. M4 E. CAEE OX THE LAM’S GOVEEXIXG 
eiiamber. In order to confine the gas in this chamber to the region where the 
electric field was uniform, a ring of ebonite C, C, Avhich projected over the edges of 
the brass plates, was Inserted. In the construction of the apparatus special 
precautions were taken to Insure that the plugs B, B pressed tightly against the 
ebonite ring. As a result ot this device, tliat portion of the electric field which was 
not uniform was entirely confined to the space occupied by ebonite, so that in this 
u ay it was rendered impossible for a discharge to occur through the gas in any but a 
uniform field. I he thickness of the ebonite ring, which could be made accurate to 
loTFo iifilliiu., determined the distance between the electrodes and consequently the 
lengtli of the discharge. The length of the discharge could be varied at will, 
thej’efore, by inserting rings of difierent thicknesses. 
I he gas Avas admitted and removed from the chamber by glass tubes sealed into 
the ebonite plugs, and these tubes were connected with the air-space by tAA'o A'er\' fine 
cliaimels leading through the eljonite ring. 
Belbre closing the discharge tube, rvliich Avas made air-tight Avith oixlinarv 
commercial soft wax, the inner surface of the ebonite ring AA^as carefully rubbed Avith 
glass ])aper to remove any cc)nducting material from its surface. 
1 he potential difieiences used in these experiments Avere obtained from a series of 
small storage cells, similar to those used in the Eeichsanstalt, Berlin. As these cells 
have a large capacity, their voltage remained constant over long intei'Amls of time, and 
as a consequence it Avas possible to make the readings AA'ith the greatest accuracy. 
The potential difierences AAmre measured by a Weston voltmeter, Avhich Avas carefully 
calibrated by means of a potentiometer furnished AA'ith a standard Weston cadmium 
element. 
Jhioughout the investigation the discharge chamber aa'us connected in series AA'ith 
a drying tube containing phosphoric pentoxide, a glass reserA'oir about 2 litres in 
A'oliime, a McLeod iiressure li'anu'e i> 
^ O o o 
iA-iiig readings accurate to xioio ^^1' millimetre. 
