120 Experiments in Electro-Magnetism. 
chine had revolved for a time, 1 found it necessary to free the re- 
volving segments, (or discs they may be called,) from oxide, even 
when it was made of silver, gold or platinum. Amalgamating the 
_ surfaces, the oxide collected with still greater rapidity. It occurred 
to me that if the interposition of oil or naphtha would not interrupt 
the current, the oxidation of the rubbing surfaces might be entirely 
prevented. On trying oil I was agreeably surprised to find that the 
current was not only not interrupted when the pressure of the metals 
was very ‘slight, but that it passed with greater certainty, and enhan- 
ced the operation of the machine six fold. It appears that oil more 
than compensates for its non-conducting property, by meeping the 
surfaces free from oxide. 
‘This discovery will prove of vast importance in the laboratory, as 
it will dispense with the use of mercury in many experiments, and 
prevent the constant necessity of amalgamating and cleaning conduc- 
tors. Having attained such an advantage in small models, | pro- 
ceeded to the construction of a large one. The revolving bars are 
a foot in length and weigh together ten pounds. ‘They are disposed 
at right angles- on the same axis, but revolving in-opposite ends of 
the cylinder of magnets. With steel magnets its power is very great ; 
but with galvanic magnets its power is sufficient to carry a machine 
for covering copper wire with cotton; and with the addition of more 
coils of wire, might doubtless be made to turn a large lathe. Now 
although it is certain that machines of this description may be applied 
to aconsiderable extent, yet it is evident that their power is limited. 
These and all other similar machines must be liable to the objection, 
that their magnetic forces cannot be made commensurate with their 
size and weight. ‘This objection I have surmounted, (as far as the- 
ory and a small model afford proof,) by the following arrangement. 
Instead of extending large bar magnets through the whole diameter 
of the circle, I have horse shoe magnets carried near to the cireum- 
ference of the circle. They are arranged on arms or radii like the 
spokes of a wheel, and both poles of each horse shoe are in opera- 
tion atonce. They each change their poles four times in each rev- 
olution, and the change is effected as before by revolving segments 
or discs. From the great success of a small model on this plan, I 
have commenced and now nearly finished an engine on a grand 
scale ; from which I expect great power. The revolving apparatus 
weighs nearly a hundred pounds. If its power should se in pro- 
portion to that of the small model, it must exceed one | 
Salem, August 15, 1837. 
