2 On Electro-Dynamic Induction. 133 
46. From the foregoing experiments, I was the more fully per- 
suaded that some new and interesting results might be obtained 
by a re-examination of my former experiments, on the phenome- 
na of the interposed plate of metal, in the case where the induc- 
tion was produced by making and breaking the circuit witha 
cup of mercury; and in this I was not disappointed. The coil, 
(Fig. 3,) being connected with a battery of ten elements, the 
shocks, both at making and breaking the circuit, were very se- 
vere; and these, as usual, were almost entirely neutralized by 
the interposition of a zinc plate. But when the galvanometer was 
introduced into the circuit instead of the body, its indications 
were the same whether the plate was interposed or not; or, in 
other words, the galvanometer indicated no screening, while, un- 
der the same circumstances, the shocks were neutralized. 
47. A similar effect was observed when the galvanometer and 
the magnetizing spiral were together introduced into the circuit. 
The interposition of the plate entirely neutralized the magneti- 
zing power of the spiral, in reference to tempered steel, while 
the deflections of the galvanometer were unaffected. 
8. In order to increase the number of facts belonging to this 
class, the last experiments were varied in several ways; and, 
first, instead of the hard steel needle, one of soft iron wire was 
placed in the spiral, with a small quantity of iron filings almost 
in contact with one of its ends. The plate being interposed, the 
“small particles of iron were attracted by the end of the needle, 
indicating a feeble, temporary development of magnetism. 
Hence the current which moves the needle, and is not neutrali- 
zed by the interposed plate, also feebly magnetizes soft iron, but 
not hard steel. 
49. Again, the arrangement of apparatus being as in paragraph 
46, instead of a plate of zinc, one of cast iron, of about the same 
superficial dimensions, but nearly half an inch thick, was inter- 
posed ; with this the magnetizing power of the spiral, in refe- 
rence to tempered steel, was neutralized ; and also, the action of 
the galvanometer was much diminished. tity 
. Another result was obtained by placing in the circuit of 
the helix, (Fig. 3d,) at the same time, the galvanometer, the spi- 
ral, and a drop of distilled water; with these the magnetizing 
power of the spiral was the same as without the water, but the 
deflection of the galvanometer was reduced from ten to about 
