16 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM, 



suppose that some new and interesting results might be obtained by a re-exa- 

 mination of my former experiments, on the phenomena of the interposed plate 

 of metal, in the case where the induction was produced by making and break- 

 ing 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 severe ; and these, as usual, 

 were almost entirely neutralized by the interposition of the 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, under the same circum- 

 stances, the shocks were neutraUzed. 



47. A similar effect was observed when the galvanometer and the magnet- 

 izing spiral were together introduced into the circuit. The interposition of the 

 plate entirely neutralized the magnetizing 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 fihngs 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 neutralized 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 interposed ; with this the magnetizing power of 

 the spiral, in reference 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 spiral, and a drop of distilled 

 water; with these the magnetizing power of the spiral was the same as with- 

 out the water, but the deflection of the galvanometer was reduced from ten to 

 about four degrees. In addition to these, the body was also introduced into 

 the same circuit; the shocks were found very severe, the spiral magnetized 

 needles strongly, but the galvanometer was still less moved than before. The 

 current of low intensity, which deflects the needle of the galvanometer in 



