302 The Rey. Dr. Rosinson on the Effect of Heat in lessening the 
racy is therefore as one to three. As to the second, it is easily shown that a 
diminution of the needle’s magnetism lessens my £, and increases the other in a 
still greater proportion: there would be no difficulty in investigating the cor- 
rection required, but it is much simpler to keep the needles in a state of satura- 
tion, which can be performed during an experiment by means of a small, flat 
helix, like that which Mr. Elias has described in the Philosophical Magazine for 
November, 1844.* 
The electrolytic intensity ¢ is easily determined on the same principles. Let 
p be the rheostat resistance which must be added to produce the same deflection 
as is observed with the voltameter included in the normal circuit; and p” that 
required to give 45° along with the voltameter, we have the equations, 
E .* teerer E wl B—€ mest 
nor+p x+r+y rtr+p rt+rt+ytp ’ 
hence 
Ep” idee ashy 
e=r— = > Y=p—p —e. 
pve 
In this process, however, each result is derived from six observations ; and there- 
fore, independent of real changes, the errors of observation must have a greater 
effect than in £, which is given by three. 
The quantity r has been determined by Mr. Wheatstone’s second method. 
The rheostat being at zero, the first copper and last zine are connected by a wire, 
whose resistance = 7 (that of the normal circuit), plunged in two small mercury 
cups drilled in their connectors: the deflection being noted, the dividing wire is 
removed, and the entire current brought to the same resistance by the rheostat. 
The added resistance = R. Simple as this is, some precautions are requisite to 
obtain uniform results. The wire is coated at its extremities with lac, so that only 
its transverse sections touch the mercury; it is of platinum, that it may be un- 
changed by use; but as this metal is easily heated by voltaic currents (which 
increases its resistance), it is lapped with fine thread, and kept wet with water. 
* That which I use has sixteen feet of copper wire, {15 thick, coiledinto a cylinder 3,7 diameter, 
and 1.2 thick, with a central aperture of 1.2 also. The needle is passed through this, having its 
poles connected by a bent piece of soft iron; and a current even one-fifth of that produced by one 
of the cells which I use, produces the maximum. It appears to me that this would be useful in 
magnetic expeditions. 
ee 
