118 L. M. Cheesman—Measurement of Electric Currents. 
After a number of experiments on the action of alternating 
currents on soft iron, with rather unsatisfactory results, a phe- 
nomenon first carefully studied by Poggendorff* was chosen 
as a basis for further study. The phenomenon referred to can 
be described as follows: if rapidly alternating electric currents 
are passed through the coil of an ordinary galvanometer, 
placed so that the plane of the windings makes an angle wit 
the magnetic meridian, a deflection takes place in such a direc- 
tion as to still further increase the angle between the plane of 
‘windings and the needle; in other words, the needle strives to 
take up a position at right angles to the plane of the windings. 
Poggendorff explained this by referring the phenomenon to 
the needle being zero. 
© examine this explanation more closely, suppose rapidly 
alternating currents, each of equal intensity, to be sent through 
the coil of a galvanometer placed so that the needle makes an 
angle of about 45° with the plane of the windings. The mag: 
netic effect of the coil on the needle can be divided into two 
components, one parallel to the magnetic axis of the needle, 
roducing no rotation but tending to magnetize it positively or 
negatively according to the direction of the current passing at 
. nt; a second at right angles to the magnetic axis 
tending to produce rotation in the needle. The effect of two 
independent therefore of the permanent magnetism of the nee 
dle. By asimilar course of reasoning it will be readily seen 
* Pogg.. Ann., xlv, 1838. 
