Natural Philosophy. ^ Electro-Magnetism. 393 
Dr Bookman next tried if the diameter of the steel needles had 
any effect on the result; and having used three of 1, 2, and 3 
lines in diameter, he found that the larger ones received the mag- 
netism more strongly than the smaller ones. The maxima of 
saturation took place between the 12th and 18th discharge. 
The greatest effect which Dr Bbckman obtained was repul- 
sive of 32°. 5 on the north pole, and 46°. 5 on the south pole. 
In determining the effect of the number of spires he obtained 
the following results. 
No. of Spites. Three Discharges. Nine Discharges. Fifteen Discharges. 
N. S. N. S. N. S. 
1 No effect was observed. 
2 A slight trace of magnetism observed. 
, 4 
9° 
10° 
13° 
14° 
13°. 5 
15°.0 
8 
13 
14 
17 
17 
17 
18.0 
16 
16 
17 
20 
21 
21.5 
22 
32 
18 
19 
22 
23 
23 
22.5 
64 
19 
20 
22 
23 
24 
24 
128 
24 
26 
28 
28 
28.5 
28.5 
174 
23 
25 
27.5 
27.5 
27.5 
27.5 
When a spiral of 58 turns had half of the spires turned to the 
right, and the other half to the left, the two extremities of the 
steel needle placed in the tube became north poles, and the two 
south poles appeared in the middle of the needle. The repul- 
sion of the north poles was 19° and 17°.5. 
Tubes of glass were thus arranged, in several ways, with spi- 
rals. The steel needles were always rendered magnetic, and 
converted into several loadstones. A steel needle was placed in 
a tube of glass, and this within another tube of white iron, an 
inch in diameter, and this last in a second tube of glass, sur- 
rounded with a spiral- wire. After the first electrical discharge, 
the tube of white iron also became magnetic, the north pole 
shewing a repulsive force of 21°. The south pole lost its magne- 
tism rapidly, and some minutes after the tube became indifferent. 
This experiment was often repeated; and, in every case, the 
steel t peedle in the interior remained indifferent ; but, when a 
tube of lead was substituted in place of the white iron, it became 
magnetic, with a repulsive force of 14° and 16°. 
A needle of nickel placed in a tube surrounded with a spiral, 
did not become magnetic after several strong electrical dis- 
charges. 
