Prof. Faraday on some Points of Magnetic Philosophy. 109 



standard or copper ball. It is easy to understand, that if the 

 globe be moved parallel to itself, but away from the magnet, in 

 a line perpendicular to the magnetic axis (as into the dotted 

 position, 3352. fig. 13), it will pass through places of weaker 

 magnetic action. Under such changes of place, the induced 

 current was weaker or stronger, according to the distance, but 

 always in the same direction. Assuming that the rotating metal 

 does give a true indication of the polarity or direction of the 

 magnetic force (3077.), the results show that the polarity 

 of the force which induces these currents, and which is the 

 magnetic force of the dominant magnet, is the same both in the 

 copper and in the iron. Other cases of the current from revolving 

 iron may be referred to in the Exp. Res. (3162.). 



3354. The bismuth globe was placed in the magnetic field. 

 If made to revolve much, with the galvanometer wire press- 

 ing against the copper equator (3352.), the latter became 

 warm by friction, and a permanent thermo-current was pro- 

 duced : this has been considered on a former occasion (3168.). 

 Its effect is easily eliminated by revolving the globe a given 

 number of times in opposite directions, observing the two 

 deflections, adding them together, and taking the half of the 

 sum for the amount of induced current in either one direction 

 or the other; for as the thermo-current is added on the one side 

 and subtracted on the other, such a process gives the real 

 amount of the induced current. When, however, the bismuth 

 sphere is revolved only five or ten times, the thermo-effect is so 

 small as to make the galvanometer deflection very little more in 

 one direction than in the other. When due attention was given, 

 the rotation of the bismuth sphere produced an induced current 

 in precisely the same direction as those obtained with the 

 copper and iron ; and so far, therefore, it indicated precisely the 

 same direction of polarity for the magnetic force then acting 

 upon and in it. 



3355. The hard steel sphere, having been previously examined 

 by a small needle and found to be unmagnetized, was placed in 

 the magnetic field. It was then revolved, and gave an induced 

 magneto-electric current in the same direction as the former 

 currents. Being removed and again examined by the magnetic 

 needle, it was found not to have received any sensible charge of 

 magnetism. 



3356. So these four metal globes indicate like polarity of the 

 magnetic force, acting upon and within them, when examined 

 thus by the magneto-electric current due to movement across the 

 lines of force. By researches described elsewhere, it is known 

 that all metals, and all bodies which are sufficiently electro-con- 

 ductors, down even to aqueous fluids, give the same direction of 



