496 Prof. J. Henry's Cotitrihutions 



the coil, in the arrangement of apparatus for the secondary 

 current, the shock was so diminished as to be inferior to one 

 produced by the arrangement for a tertiary current ; yet, while 

 with the secondary current the needle was deflected twenty-five 

 ilegrees, with the tertiary it scarcely moved more than one de- 

 gree ; and with the currents of the fourth and fifth orders the 

 deflections were still less, resembling the effect of a slight im- 

 pulse given to the end of ihe needle, 



46. With the light obtained from the foregoing experi- 

 ments, I was the moi'e fully persuaded that some new and in- 

 teresting results might be obtained by a re-examination of 

 my former experiments, on the phaenomena of the interposed 

 plate of metal, in the case where the induction 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 severe ; 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 Ijody, 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 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 neutra- 

 lized the magnetizing power of the spiral, in reference to tem- 

 pered steel, while the deflections of the galvanometer were 

 unaffected. 



48. In order to increase the number of fects 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 be- 

 ing 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 para- 

 graph 46, instead of a plate of zinc, one of cast iron, of about 

 the same superficial dimensions, but nearlj' 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. Aijother result was obtained by placing in the circuit 



