Electromagnetic Experiments of Faraday and Pliicker. 1381 
power of the gas increases continuously with the exhaustion 
which it undergoes, even when this is pushed as far as 
possible, it would be more philosophical to assume that an 
absolute vacuum is a conductor. 
XVI. On some Electromagnetic Experiments of Faraday and 
Pliicker. By 8. ToLverR Preston*. 
[Plate II. ] 
ee it might seem late in the present day to call 
in question any at all fundamental theoretic conclusions 
on the subject of magnetism deduced from experiment, yet we 
know that cases admitting of a double interpretation do some- 
times occur in experimental research ; and precisely on account 
of their extreme rarity are such cases liable to elude attention 
and to lead to misapprehensions. About the year 1869, when 
reading Faraday’s ‘Hixperimental Researches,’ such an instance 
attracted my notice, and it recurred some years afterwards 
while perusing a paper of Pliicker’s (Pogg. Ann. Bd. Ixxxvii. 
1852, 8.352: “Ueber die Reciprocitat der elektromagnetischen 
und magnetoelektrischen Hrscheinungen’’). ‘The same ambi- 
guous case occurred both to Faraday and Pliicker; and neither 
appears to have noticed the fact of its being ambiguous (or 
admitting of a double interpretation). Yet the interpretation 
taken by them, when carried out to its logical conclusions, 
appears to contradict the theory of Ampére as to the similarity 
of the helix (solenoid) and magnet in their chief properties— 
a theory which seems to have been hitherto generally accepted. 
I will at once proceed to the point in question. Faraday, 
and also Pliicker, adopted quite a peculiar view as to the 
behaviour of the system of force (“lines of force”) about a 
magnet when rotated on its axis; and they set up in this way 
(led by the experiments) a fundamental distinction of principle 
between the translatory and rotatory motion of a magnet in 
regard to the behaviour of the field of force about it. Fara- 
day himself calls his own view “singular.” The following 
are the words of Faraday relating to this point :— 
“When lines of force are spoken of as crossing a conducting 
circuit, it must be considered as effected by the translation of 
a magnet. No mere rotation of a bar-magnet on its axis 
produces any inductive effects on circuits exterior to it. The 
system of power about the magnet must not be considered as 
revolving with the magnet any more than the rays of light 
* Communicated by the Author, 
