268 BULLETIN OF THE 



into the water parallel to a line joining the two poles, a shock is 

 felt through the arms." 



The former experiment of obtaining an induction shock from 

 one room to another through a partition, was repeated on a 

 still larger scale. All the coils of copper ribbon having been 

 united in a single continuous conductor of about 400 feet in 

 length, "this was rolled into a ring of five and a half feet in 

 diameter, and suspended vertically against the inside of the large 

 folding doors which separate the laboratory from the lecture- 

 room. On the other side of the doors in the lecture-room and 

 directly opposite the coil was placed a helix formed of upwards 

 of a mile of copper wire, one-sixteenth of an inch in thickness, 

 and wound into a hoop of four feet in diameter. With this 

 arraiigement, and a battery of 147 square feet of zinc surface 

 divided into eight elements, shocks were perceptible in the tongue 

 when the two conductors were separated to the distance of nearly 

 seven feet. At tlie distance of between three and four feet, the 

 shocks were quite severe. The exhibition was rendered more 

 interesting by causing the induction to take place through a 

 number of persons standing in a row between the two con- 

 ductors." 



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 

 secondary coils: sliowing 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 con- 

 nected with a battery often 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 galvano- 

 meter indicated no screening, while under the same circumstances 

 the shocks were neutralized. A similar effect was observed 

 when the galvanometer and the magnetizing helix were together 

 introduced into the circuit. The interposition of the plate en- 

 tirely neutralized the magnetizing power of the helix (in refer- 

 ence to tempered steel) while the deflections of the galvanometer 

 were unaffected." The induction currents of the third, fourth, 

 and fifth orders, were found to be of considerable " 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 neutralized by a me- 

 tallic plate interposed between the coils.* 



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



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