J74 dr. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIII.) 
the time be considered constant, and the experimental helix may at any moment be 
connected with the galvanometer without any current appearing 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 experi- 
mental 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 
derived 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 experimental 
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 re- 
newing 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 fora still shorter period; 
and when the magnet has been used at short intervals for some time, it seems capa- 
ble of receiving its maximum power almost at once. In every experiment it is neces- 
sary 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 pecu- 
liar action of the bismuth or other body under investigation. 
265 1 . The galvanometer employed was made by Ruhmkorff and was very sensible. 
The needles were strengthened in their action and rendered so nearly equal, that a 
single vibration 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 quite sufficient to intercept a 
feeble current. 
2652. In order to bring the phenomena afforded by magnetic 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 under- 
stood, 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 piece of bismuth or 
other body whose effects were to be observed and compared. 
2653. A thin glass tube, of the given size (2643.), 5^by-| inches, was filled with a 
saturated solution of protosulphate of iron, and employed as the experimental core : 
the velocity given to the machine at this and all average times of experiment was 
