166 WRITINGS OF JOSEPH HENRY. [1840 



of the secondary current; and this is not on account of the 

 diminishing power alone of the successive inductions, as will 

 be evident from the following experiment : By raising the 

 helix from the coil, in the arrangement of the 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 degrees, with the tertiary it moved 

 scarcely more than one degree; and with the currents of the 

 fourth and fifth orders the deflections were still less, resem- 

 bling the effect of a slight impulse given to the end of the 

 needle. 



46. With the light obtained from the foregoing experi- 

 ments, I was the more fully persuaded that some new and 

 interesting results might be obtained by a re-examination of 

 my former experiments, on the phenomena of the inter- 

 posed 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 galva- 

 nometer was introduced into the circuit instead of the body, 

 its indications were the same whether the plate was inter- 

 posed 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 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 

 filings almost in contact with one of its ends. The plate 



