288 
MR. joule’s introductory RESEARCH ON THE 
To measure the electrical currents, I employed a galvanometer of tangents, the 
needle of which, half an inch long, carried a glass index over a divided circle 6 inches 
in diameter. This instrument was furnished with a coil of sixteen circumvolutions 
of 1 foot diameter, which could be exchanged for a single circle of 1 foot diameter 
when the intensities to be measured were very considerable. It was ascertained by 
experiment that the tangent of deflection by the former coil was exactly sixteen times 
that of the latter when the same intensity of current was employed. For convenience 
sake I have reduced all the observations to the latter standard; the unit current 
being therefore that which, passing through a circle 1 foot in diameter, is able to 
deflect the needle through 45°. 
The amount of magnetism induced in a bar was ascertained by placing it vertically 
with its lower end at a distance of 6 or 12 inches from a magnetized needle i^ths of 
an inch long and 4 -oth of an inch in diameter, suspended by a filament of silk, and 
having a fine glass index traversing over a graduated circle 6 inches in diameter. 
The force of torsion of the filament was found to be so trifling, that the tangents of 
the deflections of the needle conld be taken as representing, without sensible error, 
the magnetism of the bar. Observations with so small a needle were made with 
great facility, the pointer moving steadily up to and attaining a new angle of deflec- 
tion in eight or ten seconds after the electrical circuit was completed, the resistance 
of the air to the motion of the pointer being such as to prevent the smallest degree of 
oscillation. This resistance, however, of the air, so useful in bringing the needle 
speedily to rest, renders it necessary to guard carefully against any irregularity of the 
temperature of the case in which it is enclosed. A ray of sun-light would speedily 
occasion a deflection of several degrees* ; and I found that the heat of the hand held 
over a part of the thick glass case 45° in advance of the pointer was sufficient, after 
penetration through the glass, to produce a current of air causing a steady deflection 
of no less than 30°, a deflection which subsided with extreme regularity and' great 
slowness after the hand was removed. I would suggest that this circumstance points 
to the means of constructing a new and exceedingly sensible thermometer which 
would be valuable in many researches, particularly those on the conduction of heat. 
Previously to employing electric currents, I made some experiments simply with a 
view to ascertain the inductive power of the earth’s magnetism on the bars ; and in 
which the action on the suspended needle was observed both at the distance of 
12 and 6 inches, in order to determine the influence of distance for the convenience 
of future reductions. Having noticed the deflection produced by any bar, it was 
reversed and the observation repeated, the sum of the tangents of deflection showing 
the total effect produced on the magnetism of the bar by its reversion. I may here 
remark, that both ends of the pointer of the needle were invariably observed, though 
to save unnecessary detail the tangent of the mean is only given. 
* Dr. Tyndall has drawn attention to the importance of guarding against these effects of heat on a deli- 
cately poised needle. Philosophical Magazine, 4th series, vol. iii. p. 127. 
