434 
DR. O’SHAUGHNESSY’S EXPERIMENTS. 
The next remarkable property of these soft 
iron electro -magnets is, the instantaneous 
change of their poles on changing the direc- 
tion of the galvanic current. If the ends of 
the spiral in contact with the galvanic plates 
be made to change places, that which was 
originally in contact with the zinc being 
brought to the copper, and vice vers^, the 
poles of the temporary magnet are changed 
on the instant too : that which was north 
becoming south, that which was south be- 
coming north. Though itself is not quicker 
than the velocity w'ith which this change is 
accomplished, even in the largest bars of 
iron which have yet been made the subject of 
experiment. 
These facts are well known to all students 
in physical ''science. They have been esta- 
blished for some years, and are mentioned here 
but in connexion with ulterior matters. On 
observing and reflecting on them, it is impos- 
sible to avoid the impression, that, by appro- 
priate mechanical contrivances, this enor- 
mous and easily generated power could be 
made available as a mechanical force. But 
the difficulties which beset the attempt are 
many and important. In the first place, 
though the sustaining power of the electro- 
magnet be immense, the force operates 
through such a small distance, that the mag- 
net which would hold up one hundred pounds, 
would not lift one pound at the distance of 
two inches — nay, at one inch. Here it is 
true we have the magnetic attraction in 
antagonism with the attraction of gravita- 
tion, and it is but the difference of the two 
foi’ces which the electro -magnet is exei'ting; 
nevertheless, even when we exclude this 
counteracting force, as in arrangements 
afterwards to be described, the space through 
which the electro- magnet attracts is very 
limited, and varies according to the object on 
which its attraction is exerted. Thus with 
soft iron the distance is exceedingly minute ; 
the poles of another electro-magnet are 
drawn from a greater distance ; and the 
poles of a permanent steel magnet through 
greater still. 
The second obstacle we encounter is so well 
described by Mr. M’Gauley in his paper pub- 
lished in the Appendix*, that I shall advert to 
it but briefly hei-e. It is the power which 
the poles of a strong electro-magnet have 
of superseding the polarity of a weaker one. 
'1 hus, if we place the poles of a horse-shoe 
electro -magnet in contact with those of a 
weaker one, so that north is opposed to 
south respectively — if we change the poles of 
the strong magnet by reversing the direction 
of its exciting galvanic current, instead of 
the strong repelling the weak, it continues to 
attract the poles of the weak — these having, 
in fact, been reversed by the mere change of 
the strong one alone. 
To overcome these difficulties, and to pro- 
cure a machine moved by soft iron electro- 
magnets, several attempts have recently been 
made. I have collected and inserted in the 
Appendix every notice 1 could find on the 
subject. Of these Mr. M'Gauley’s contri- 
vance of the magnetic pendulum is infinitely 
the best : that of Signor Botto being on the 
same principle, but much less skilfully ar- 
ranged*. 'Ihe American modelf is so obscure- 
ly noticed, and the paragraph relating to it so 
vei’y like a wild newspaper exaggeration, that 
it is impossible to form any precise idea as 
to its construction, its advantages, or its 
power. For Mr. M’Gauley’s model I may 
refer to his own clear and accurate descrip- 
tion. Its chief defect is the mode in which 
the power is applied. Though the pendulum 
moves through a wide arc, still it is acting 
merely as a pendulum through the whole of 
its vibration, except in the very small space, 
where the magnetic attraction is exercised. 
The force acquired by the pendulum in falling 
through half its arc, is necessarily expended 
in lifting it through the other half. In the 
whole of the intervening space, then, between 
the limits of the magnetic attractions at 
either side, no available force is in existence. 
Again, the motion, being a reciprocating one, 
has to be converted by a crank and a fly 
wheel into the rotatory one required for 
locomotive engines. Thus a great expendi- 
ture of force is occasioned, which, as I shall 
presently shew, may be economized by a 
dilferent mechanical arrangement. 
On commencing my experiments in July 
last, my principal object was, if possible, to 
apply the force directly to the moving of a 
wheel. Could this be accomplished, it seem- 
ed to me that we would use the whole of the 
magnetic force, unopposed by terrestrial gra- 
vitation — that we would act at the greatest 
possible mechanical or lever advantage — and 
that should one wheel succeed, a series might 
be so arranged together, that the maximum 
of several forces might be made to co-operate, 
so as to render a number of the small spaced 
magnetic powers (say 12 powers at half an 
inch), equivalent to one power acting uniform- 
ly through the whole space, say six inches. 
The first set of experiments was instituted 
for the determination of the circumstances 
under which the poles of one. electro-magnet 
overpower those of a second. A pair of semi- 
circular bars of equal size were provided, 
wound with spirals, each spiral connected 
with a separate galvanic battery, and the 
attracting poles of the semi-circles brought 
into contact: See fig. 1. 
On changing the poles of B, by moving the 
ends of the spiral wire from the copper to the 
zinc element and vice versh, the poles of the 
magnet A were overpowered and changed 
also, without disturbing their loires, so that the 
semi-circles continued to attract each other 
as before. Now in this experiment all cir- 
cumstances connected with the two semi- 
circles were as equal as possible. The bat- 
teries were of the same size and same strength, 
bars of the same weight and dimensions, cut 
of the same piece of iron, the spirals were of 
the same length, &c., still the magnet B 
overcame the poles of A. By reversing the 
experiment, that is, removing the wires of A 
* Vide our Review, page 335. 
* Vide our Review, page 337. 
+ Vide our Review, page 401. 
