of Electro-Magnetism by Heat. 53 
In the beginning of this paper it was mentioned that all the 
metals possess the power of exhibiting electro-magnetism by heat, 
but in different degrees. Even when these are the greatest, they 
are much inferior to what can be readily produced in the common 
galvanic apparatus; it was consequently necessary, in detecting 
and forming a comparison of their relative powers, to use the 
delicate instrument described in the First Volume of our Transac- 
tions*; by which some, though not in all cases, an accurate 
measure of them could be obtained. For the more minute effects 
a compass was employed in the galvanoscope, having its terrestrial 
magnetism neutralized, which gave a deviation of from 10° to 
20° with two disks of zinc and copper of 4> inches diameter, excited 
by spring water, and which was readily sensible to the galvanic 
action of zinc and copper wires, excited by nitric acid, whose 
diameters were less than 5, and whose excited surfaces were con- 
sequently between ;; and 3, of an inch. With this instrument, 
two rods of zinc and copper of 3 inch diameter and apart, excited 
by equal parts of muriatic acid and water, gave a deviation of 
40°; a bar of bismuth 4+ inches long by + broad, and ? thick, 
gave 70° of deviation at the melting point of the bismuth, and 
10° when the difference of the temperatures of its extremities was 
12° of Fahrenheit. The slip of palladium before mentioned, which 
was 23 inches long by % broad, and weighed 35 grains, gave a 
deviation of 70° positive with silver, and 10° negative with platina 
wires, when heated red hot by a spirit lamp: a slip of platina 
* The spiral wire of the instrument was originally of copper, but silver wire of the same 
diameter is more efficacious. If the spiral parallelogram be vertical, and the wire of 
silver of = inch diameter, it acts very powerfully, and presents little or no impediment 
to a view of the needle; for this reason I prefer the vertical to the horizontal form of the 
spiral. The needle is neutralized by placing a powerful magnet North and South on a line 
with its centre; and another, which is much weaker, East and West at some distance 
above it: by means of the first, the needle is placed nearly at right angles to the meri- 
dian, and the adjustment is completed by the second. 
