154 MR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 
effects, curious as they appear when witnessed, are immediately referable to 
the action of single wires (40. 109.). 
113. Although the experiments with the revolving plate, wires, and plates 
of metal, were first successfully made Avith the large magnet belonging to the 
Royal Society, yet they were all ultimately repeated with a couple of bar mag- 
nets tAvo feet long, one inch and a half Avide, and half an inch thick ; and, by 
rendering the galvanometer (87.) a little more delicate, with the most striking 
results. Ferro-electro-magnets, as those of Moll, Henry, &c. ( 57 .), are very 
powerful. It is very essential, when making experiments on different sub- 
stances, that thermo-electric effects (produced by contact of the fingers, &c.) 
be avoided, or at least appreciated and accounted for ; they are easily distin- 
guished by their permanency, and their independence of the magnets. 
114. The relation which holds between the magnetic pole, the moving wire 
or metal, and the direction of the current evolved, i. e. the law which governs 
the evolution of electricity by magneto-electric induction, is very simple, 
although rather difficult to express. If in fig. 24. PN represent a horizontal 
wire passing by a marked magnetic pole, so that the direction of its motion 
shall coincide with the curved line proceeding from below upwards; or if its 
motion parallel to itself be in a line tangential to the curved line, but in the 
general direction of the arrows ; or if it pass the pole in other directions, but 
so as to cut the magnetic curves * in the same general direction, or on the 
same side as they would be cut by the wire if moving along the dotted curved 
line ; — then the current of electricity in the wire is from P to N. If it be carried 
in the reverse directions, the electric current Avill be from N to P. Or if the 
wire be in the vertical position, figured P' N', and it be carried in similar 
directions, coinciding Avith the dotted horizontal curve so far, as to cut the 
magnetic curves on the same side Avith it, the current will be from P' to N'. 
If the wire be considered a tangent to the curved surface of the cylindrical 
magnet, and it be carried round that surface into any other position, or if the 
magnet itself be revolved on its axis, so as to bring any part opposite to the 
tangential Avire, — still, if afterwards the wire be moved in the directions indi- 
* By magnetic curves, I mean the lines of magnetic forces, however modified by the juxtaposition 
of poles, which would be depicted by iron filings ; or those to which a very small magnetic needle 
would form a tangent. 
