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 with a 

 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 mdicated 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. 



48. 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. 



50. 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 



