1822.] Cambridge Philosophical Transactions, Part II. 291 
in decomposing water, or fusing métallic wire; both these 
methods he considers as presenting considerable practical diffi- 
‘culties, either when the quantity of electricity is small, or its 
intensity low. The discoveries of Oersted have, however, 
enabled Prof. Cumming to construct two instruments, one for 
discovering, and the other for measuring, galvanic electricity ; 
‘and he is of opinion that their delicacy and precision scarcely 
admit of limitation. The construction of the first instrument 1s 
alluded to as having been already communicated to the Society ; 
‘the following experiment is adduced in support of the opimion 
entertained of the delicacy of the instrument: A wire of zinc 
and another of platina, each 1-10th of an inch in diameter, were 
coated with sealing wax, so as to have merely their extremities 
exposed: on immersing them in a dilute acid, the circuit being 
-at the same time completed through the galvanoscope, the needle 
deviated so decidedly, as to leave no doubt that a visible effect 
~yould have been produced by wires of less than half of the 
dimensions of those employed; as the compass used, though 
small, was not delicate. Prof. Cumming is of opinion, that the 
electricity developed by two metallic surfaces, each one 1-500th 
-of a square inch, may be detected, and their relations to each 
other ascertained by this instrument. 
Prof. Cumming states, that although he has not had sufficient 
{eisure to form so complete a series of the electric relations of the 
metals towards each other, as this instrument is capable of doing, 
“yet there are two instances which he mentions as being remark- 
able. On using two disks, one of iron, the other of steel, there 
“was produced a decided deviation ; since then the only differ- 
‘ence in the metals arises from an alloy of from 1-60th to 1-100th 
part of the whole, it appears that this is sufficient to alter their 
electric relations. The powerful affinity of potassium for oxygen 
‘made it highly probable that in the galvanic circuit, it would 
become strongly negative with all the metals. On the first trial 
‘with disks of potassium and zinc, the potassium took fire before 
the effect could be observed; this difficulty was afterwards 
obviated by alloying it with mercury; on making the contact 
‘the needle deviated through nearly a right angle: the same 
effect was produced by copper. 
With the assistance of the late Dr. Clarke and Mr. Lunn, the 
‘magnetic effects of atmospherical electricity were tried by a wire 
of about 100 yards in length, connected with a kite; a steel nee- 
dle inclosed in a spiral wire was readily magnetized, but no 
deviation was caused in a compass placed beneath it. 
Prof. Cumming concludes that. galvanic magnetism is most 
readily made sensible by the deviation it causes in the compass 
needle ; but the electrical by its power of communicating per- 
‘manent magnetism. 
For various other curious particulars, we must refer to the 
paper. 
u 2 
