254 MEMORIAL OF JOSEPH HENRY. 



The second section of the memoir is mainly occupied with details 

 of experiments on the screening effect of conducting plates (of non- 

 magnetic metals) when interposed between the primary and second- 

 ary coils: showing remarkable contrasts in the "quantity" and 

 "intensity" classes of galvanic effects. When the annular spool 

 or helix (of nearly one mile of copper wire) was employed with the 

 large spiral coil of copper ribbon, "the coil 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 instead of the body, was introduced into the circuit, 

 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. 

 A similar effect was observed when the galvanometer and the mag- 

 netizing helix were together introduced into the circuit. The 

 interposition of the plate entirely neutralized the magnetizing 

 power of the helix (in reference to tempered steel) while the deflec- 

 tions of the galvanometer were unaffected." The induction currents 

 of the third, fourth, and fifth orders, were found to be of consid- 

 erable "intensity;" magnetizing steel needles, giving shocks, not 

 being interrupted by a drop of water placed in the circuit between 

 the ends of the severed wire, and yet being screened or neutral- 

 ized by a metallic plate interposed between the coils.* 



A continuation of the memoir was read before the Philosophical 

 Society November 20th, 1840, discussing further the theoretical 

 differences between an initial or an increasing galvanic current, and 

 a decreasing or an arrested current, in producing the phenomena of 

 induction. On the same occasion Henry described "an apparatus 

 for producing a reciprocating motion by the repulsion in the consec- 

 utive parts of a conductor through which a galvanic current is 

 passing." About ten years before, he had devised the first electro- 

 magnetic engine (operating by intermittent magnetic attractions and 

 repulsions); and now he had contrived the first galvanic engine, 

 operating by the analogous intermittent attractions and repulsions 

 of the electric current, f 



* Trans. Am. Phil. Soc. June 1840, vol. viii. (n. so art. i. pp. 1-18. 

 t Proceedings Am. Phil. Soc. Nov. 20, 1840, vol. i. p. 301. 



