94 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
The expansion which arises when a discharge takes place at various distances and 
pressures:— 
Pressure. 
Depression. 
Ratio of 
increased to 
normal pressure. 
Current. 
M. 
m.m. 
W. 
ratios. 
985,000 
None 
388,026 
16 
1-054 
0-02881 
1-07 
251,711 
17 
1-089 
0-04060 
1-51 
187,631 
19 
1-133 
0-04474 
1-66 
148,157 
19 
1-169 
0-03459 
1-28 
130,789 
21 
1-211 
0-03071 
1-14 
113,026 
22 
1-256 
0-03258 
1-21 
94,210 
22 
1-307 
0-02693 
1-00 
86,184 
22 
1-336 
0-02693 
1-00 
84,737 
20 
1-310 
0-03071 
1-14 
The arc in air between two spherical surfaces, 3 inches radius, and 1*5 inch diameter 
at various distances and pressures, with 10,960 cells. Temp. 15 c- 2 C. 
PLATE 10. 
(All the representations are from drawings.) 
Distance 0 16 inch (the current would not pass at 0T65 inch), pressure 761 m.m., 
1,001,317 M, current 0'02371 W, no depression apparent on making connexion with 
the battery, fig. 1. 
Distance X 2 = 0*32 inch, pressure 272T m.m., 358,027 M, current 0*02456 W, 
no depression, fig. 2 ; the central spindle is broken up into nebulosities. 
Distance X 3 = 0*48 inch, pressure 18L3 m.m., 238,553 M, current 0*02209 W, 
depression 0*25, total pressure 18L55 m.m., ratio of increased pressure 1'0002, fig. 3, 
one detached luminosity near negative. 
Distance X 4 = 0*64 inch, pressure 149*8 m.m., 197,111 M, current 0*02131 W, 
depression 1 m.m., total pressure 150 - 8 m.m., ratio of increased pressure 1*007, fig. 4 ; 
the discharge no longer occurred at the shortest distance, and moreover took a curved 
formation, and was of unequal brightness in the central portion. 
Distance X 5 = 0*80 inch, pressure 140 m.m., 184,211 M, current 0*02209 W, 
depression 3'5 m.m., total pressure 143'5 m.m., ratio of increased pressure 1*025, 
fig. 5 ; the discharge was curved, therQ was a small dark space near the negative on 
which there was a glow. 
Distance X 6 = 0*96 inch, pressure 117*9 m.m., 155,131 M, current 0*02249 W, 
depression 2*5 m.m., total pressure 120*4 m.m., ratio of increased pressure 1*021 ; 
fig. 6. The discharge was pointed towards the negative. 
Distance X 7=1*12 inch, pressure 118*4 m.m., 155,789 M, current 0*02209 W, 
