868 On the Duration of the Spark of an Electric Discharge. 
towards the ball f, establishing a metallic communication between 
ABCD and FE, and at the same instant a spark passed between 
the two knobs of the spark-micrometer. The amount of residue 
was then determined by the electrometer. The results I ob- 
tained are collected in the following Table,in which the amount 
of residue is represented by R, that of the charge by L:— 
| 
Name of the _ | Deviation of the magnetic needle 
| 
Distance between | substance placed | Value of 
R 
excitator. discharge. | discharge. 
| 
__ 
the knobs of the between the i aa ee : 
micrometer. | branches of the | Before the After the & 
0°88 millim. | Hempen cord. | 52 20 
5 18 0-117 
+ 7 Brass wire. ie 3 18 0-073 
+ at Hempen cord. +i 4 42 0-103 
1 millim. | Hempen cord. 64 26 vor 0:137 
7 . Brass wire. 9 3 56 0-076 
“ Es Hempen cord. 5 6 56 0-134 
From this Table it is obvious that the residue is more con- 
siderable when the body through which the discharge takes place 
offers more resistance. | 
- 5. Itis possible that the above theory may not at first meet 
with universal assent. I trust, however, that those physicists 
who hesitate to admit it will find their doubts dispelled on con- 
sidering the following experiment :—A BC D is a hollow cylin- 
der closed at the end C D and fitted at the other end AB witha 
sucker-valve opening outwards. Near the end CD there isa 
slide GH pierced with a large opening at I. The anterior por- 
Gq 
ot ald 
AURA RARE! ure re 
eee? 2 ee eee Se z£ 
F D 
HL 
tion ABEF of the tube must be supposed exhausted of air, 
while in the remaining part E F C D the air is ina state of great 
compression. Now suppose the slide GH suddenly depressed 
until the centre of the opening I coincides with the axis of the 
cylinder. It is evident that the air enclosed in E FC D will im- 
mediately begin to spread throughout the anterior portion of the 
tube until it reaches AB, where, if its tension be sufficiently 
great, it will commence escaping from the tube. If the air has 
taken z" to arrive at A B, it will of course begin to escape z" after — 
the slide has been depressed; but no one will imagine that the 
