above it and that this action is stopped by an interposed screen of 

 magnetic material or thick plates of copper, silver, or zinc. Also, 

 Faraday had shown that the rotation is due to inductive rather than 

 magnetic action, the rotating magnet causing induced currents in the 

 suspended body. 



Henry did not know of the experiments on screening which Faraday 

 had made and which were the subject of a paper read June 21, 1838, 

 and published in the Philosophical Transactions for that year.*^ In 

 this work Faraday found that a copper plate interposed between a 

 primary and a secondary coil had no effect on the induction as indicated 

 by a galvanometer, apparently of the slow-period type. Under this 

 condition of measuring, the induced currents in the shields, which set 

 up a flux opposing that of the primary coil, die out before the galvanom- 

 eter response is well under way, so that the deflection is dependent 

 only on the total change of flux in the secondary coil and hence sub- 

 stantially independent of the screen. 



Henry, on the other hand, using shocks, which are indicative of 

 instantaneous or peak voltages, as the measure of induction, observed 

 the important principle of shielding, namely, that an interposed non- 

 magnetic metallic conducting screen prevents sudden changes of flux 

 through the screen. The "measuring instrument" in this case being 

 instantaneous, the opposing effect of the currents induced in the shield 

 is observed. Henry communicated these results to the American 

 Philosophical Society November 2, 1838, a report of which, published 

 in 1839, stated that a part of the paper: 



" relates to the effect of interposing different substances between the conductor which 

 transmits the current from the battery, and that which is arranged to receive the 

 induced current. All good conducting substances are found to screen the inducing 

 action, and this screening effect is shown, by the detail of a variety of experiments, 

 to be the result of the neutralizing action of a current induced in the interposed body." ^^ 



Henry, on later learning of Faraday's results and of the then apparent 



discrepancy between those and his own, investigated the subject 



further with additional experiments, and from the experimental data 



and theoretical reasoning arrived at a fair reconciliation of the various 



observations.^^ 



Speaking of the phenomena of circulating currents induced in an 



interposed shield, Henry said: 



"Also the same principle appears to have an important bearing on the improve- 

 ment of the magneto-electrical machine: since the plates of metal which sometimes 

 form the ends of the spool containing the wire, must necessarily diminish the action, 



^- Experimenlal Researches in Electricity by Michael Faraday, Vol. 1, paragraphs 

 1709-1736. 



33 SW, Vol. 1, p. 106. 



" SW, Vol. 1, pp. 169-188. 



18 



