160 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
and x 4'3 75 inches, between x and y 5‘75 inches, and between y and 2 4‘25 inches; 
the length of the constricted part being 3'75 inches, and its diameter 0'125 inch. The 
distances for tube 155 are respectively between w and x 5'25 inches, between x and y 
2 '3 7 5 inches, and between y and z 5 ’25 inches ; the constricture in this case is a glass 
diaphragm 0‘03 inch thick with a hole 0'125 inch in diameter. 
The following results were obtained from observations made January 25th, 1878 : — 
Terminal 2 + ; terminal w — (to Earth). 
Tube 154. 
Tube 155. 
Difference of potential between a and y 
75 
81 
„ ,, y „ x (the constriction) 
00 
05 
rH 
32 
,, „ X „ IV 
118 
146 
„ „ x „ w 
331 
259 
The diaphragm in tube 155 being 0‘03 inch in thickness, and in tube 154 the 
constriction being 3 '7 5 inches long, the one is 125 times longer than the other, but 
the ratio of the differences of potential between x and y in the two cases is only 4 ‘31. 
It is evident, therefore, that the main effect is due to the simple constricture of 
the tube. 
Among the tubes depicted in the diagram fig. 37, only the following require any 
special allusion being made to them in this place : — 
Tube 145 has at the right hand a chamber for holding an absorbent substance 
(spongy palladium, charcoal) ; the vertical tube is for connexion with the pump, 
and the left hand small tube for connexion with the supply of gas ; by using spongy 
palladium with hydrogen, a vacuum has been obtained in which 11,000 cells could not 
produce a current.* 
Tube 19 is one of the so-called induction tubes, the tube enclosing one of the 
wires ending in a closed chamber (a globe) surrounded by that portion of the tube 
enclosing the other terminal, so that there is not any continuous gas space from 
one terminal to the other. 
Tube 81 has a carbonic acid vacuum, with an absorption chamber containing hydrate 
of potash, which produces so good a vacuum that the current from 11,000 cells will not 
pass continuously, but there is a flash of light on making contact in one direction but 
not in the other. 
Tube 143 is a tube so thoroughly exhausted that a spark from an induction coil will not 
pass between two terminals only 0‘1 of an inch apart, although of sufficient tension to 
jump across the wires outside the tube several inches distant ; this communicates with 
a radiometer. 
* Gassiot (Phil. Trans., 148-150) describes several “ non-conducting vacua ” produced by absorbing 
substances (caustic potasb, and chloride of calcium). 
