470 
LATEST INTELLIGENCE ON THE ELECTRO-MAGNET. 
increase of power beyond that obtained from 
the small magnetic arcs represented in the 
figure. 
Induction is certainly a cause, but not the 
sole cause oi the increased power ; there are 
other causes, as yet unexplained, which I 
trust may appear satisfactory to those who 
may peruse a paper which 1 am now prepar- 
ing on this highly interesting subject ‘. suffice 
it here to say, that in the future construction 
of the instruments in question, magnetic arcs 
in connexion with vertical bar-magnets should 
decidedly be used in preference to any otlier 
form or mode of construction at present 
known; and I would strongly advise any 
person who happens to have a machine of the 
horse-shoe form to cut off the bend as indicat- 
ed in the annexed figure ( Plate 2, fig.l 1 ) and 
re apply the same oi other pieces of the same 
size magnetized, for by so doing it wilt be 
founrl that a vast increase of power will be ob- 
tained. I liave thrown out these hints in the 
hope that lliey may lead to still greater im- 
provements in the mode of developing the 
povveis of combined magnets. In concluding 
this subject it may be well to observe that 
with my improved magnetic machine I have 
charged a l^eyden jar, and obtained by the 
same means various others results similar to 
those obtained from the action of the com- 
mon elecliical machine. 
In conclusion I would add, that in the 
various experiments I have made in regard to 
the best modes of developing and sustaining 
voltaic electricity, I have found that caout- 
chouc, or Indian-rubber, may be used with 
great advantage for insulation. I have applied 
it in place of glass in my intensity-sustaining 
battery ; and as it can be naade to adhere to 
the copper and may be laid oii as thin as 
common letter-paper, a combination of plates 
or cylinders may be brought so close together 
as to occupy only a third of the space filled by 
a similar combination in the batteiies at pre- 
sent used. In my intensity-battery, from the 
advantages derived from bringing the me- 
tallic cylinders as close as possible, this mode 
of insulation is most convenient and satisfac- 
tory. 
I am. Gentlemen, yours, &c. 
Fred. W. Mullins. 
Bouse of Commons, July 1, 1836. 
ON THE CAUSE OF THE REMARK- 
ABLE DIFFERENCE Bl/l'WEEN 
THE ATTRACTIONS OF A Pl RMA- 
NENT AND OF AN ELECTRO- 
MAGNET ON SOFT IRON AT A 
DISTANCE. 
By the Rev. William Ritchie, L. L. D., 
F. R. S,, 
Professor of Natural Philosophy in the Royal 
Institution and in the University of London* 
As soon as the electro-magnet was con- 
structed and employed to illustrate the im- 
» Communicated by the Author, 
mense magnetic power communicated to ; 
soft iron, it must have been observed that its ’ 
attraction for iron filings or pieces of soft iron j 
at a distance was much less than that of a ! 
permanent magnet of equal lifting power i! 
This peculiar property rendered the electro" ' 
magnet not well suited tor magnetic induc- 
tion at a distance ; and hence, after a few 
unsuccessful trials to substitute it for the 
permanent magnet in my apparatus for con-* 
tinned rotation, it was lone since abandoned. 
In a short paper by Mr. Rainey in the 
last Number of this Journal, p. 72, ihe fact 
is stated, and an explanation attempted to be 
given of this peculiarity ; bul 1 am afraid the 
explanation will not be found in accordance 
with the present stale of the science. This \ 
stibjecl having engaged my attention some !| 
years ago, I liad several times commenced a j 
paper intended (or the Philosophical Maga- jj 
zine, but other more pressing subjects prevent- i 
ed me from finisliing it. As the fact is a ne- j 
cessary consequence of the properties of ! 
electro-maguets which I formerly made pub- j 
lie in your Journal, I take the iiferty of i 
sending you the present investigation, wldch jj 
may be regarded as the completion of my for- I 
mer paper. |[ 
Expei iment 1. Suspend a piece of soft iron, | 
C D, at the extremity of a slender delicate 
balance of light wood; place a permanent 
lioi se-shoe magnet below it, and ascei tain its 
attractive force, by weights put into the scale ! 
G, when it is in contact, and also when it is 
removed to different distances from the soft 
iron- Remove the permanent magnet, and 
substitute a very short electro-magnet of equal 
lifting power. Remove it to the same distan- 
ces as before, and the attractive power will di* 
minish very rapidly compared with that of the 
permanent steel magnet- (See Plate 2, fig. 9). 
Exp. 2. Instead of the short electro-magnet 
substitute a very long one (one of two or three 
feet long, for example,) and of equal carrying 
power ; remove it to the same distances and 
asceitain its attractive power, and it will be , 
found (hat its attraction for the lifter at these I 
distances will ?iot diminish so rapidly as that 
of the short one* The longer the electro-mag- 
net becomes, the more does it approach to 
the character of the permanent magnet in all | 
its pioperiie-. . 
Exp. 3- Instead of making the electro- | 
magnet of soft iron, make it of hard iron or un. if 
tempered steel ; repeat the preceding experi- li 
ments. and its attractive power at a distance 
compared with its lifting power will he much 
greater than in the case of the electro-magnet 
of soft iron* 
These facts, which as far as I know, have i 
not belore been published, will enable us to 
account for this properly on principles pre- I 
viously recognised. 1 he perlect equality of ! 
action and reaction must 1)6 found to exist in | 
this case as well as in every other in which j 
/orce ofany kind is concerned. The electii- 
city which has been decomposed and arranged ; 
in the soft iron in the peculiar manner which 
constitutes magnetism, cannot decompose j 
and arrange the electricity belonging to the 
lifter without suffering a corresponding dimi- 
