92 Dr. Faraday's Experimental Researches in Electricity : 



ing there. But if the magnet be employed in the excited 

 state, certain important precautions are necessary; for upon 

 connecting the magnet with the battery and then connecting 

 the experimental helix with the galvanometer, a current will 

 appear at the latter, which will, in certain cases, continue for 

 a minute or more, and which has the appearance of being de- 

 rived at once from that of the battery. It is not so produced, 

 however, but is due to the time occupied by the iron core in 

 attaining its maximum magnetic condition (2170. 2332.), 

 during the whole of which it continues to act upon the expe- 

 rimental helix, producing a current in it. This time varies 

 with several circumstances, and in the same electro-magnet 

 varies especially with the period during which the magnet has 

 been out of use. When first employed, after two or three 

 days' rest, it will amount to eighty or ninety seconds, or more. 

 On breaking battery contact and immediately renewing it, the 

 effect will be repeated, but occupy only twenty or thirty 

 seconds. On a third intermission and renewal of the current, 

 it will appear for a still shorter period ; and when the magnet 

 has been used at short intervals for some time, it seems ca- 

 pable of receiving its maximum power almost at once. In 

 every experiment it is necessary to wait until the effect is shown 

 by the galvanometer to be over ; otherwise the last remains 

 of such an effect might be mistaken for a result of polarity, or 

 some peculiar action of the bismuth or other body under in- 

 vestigation. 



2651. The galvanometer employed was made by Ruhm- 

 korff and was very sensible. The needles were strengthened 

 in their action and rendered so nearly equal, that a single vi- 

 bration to the right or to the left occupied from sixteen to 

 twenty seconds. When experimenting with such bodies as 

 bismuth or phosphorus, the place of the needle was observed 

 through a lens. The perfect communication in all parts of 

 the circuit was continually ascertained by a feeble thermo- 

 electric pair, warmed by the fingers. This was done also for 

 every position of the commutator, where the film of oxide 

 formed on any part by two or three days' rest was quitesuffi- 

 cient to intercept a feeble current. 



2652. In order to bring the phenomena afforded by mag- 

 netic and diamagnetic bodies into direct relation, I have not 

 so much noted the currents produced in the experimental 

 helix, as the effects obtained at the galvanometer. It is to be 

 understood, that the standard of deviation, as to direction, has 

 always been that produced by an iron wire moving in the 

 same direction at the experimental helix, and with the same 

 condition of the commutator and connecting wires, as the 



