264 BULLETIN OP THE 



ent circuit : and during the seven years in which galvanic induc- 

 tion liad been known, no physicist ever thouglit of making 

 the trial. Theoretically it might perhaps have been inferred, 

 if such tertiary inductiou had any existence, as it would be 

 coincident not with the instantaneous secondary induction, but 

 with the initiation and termination of such momentary current, 

 and hence in opposite signs — separated by an inappreciable 

 interval of time, that the whole phenomenon would probably be 

 entirely masked by a practical neutralization. The experiments 

 of Henry fully established, however, the new and remarkable 

 result of a very appreciable tertiary current. By connecting 

 the secondary coil with another at some distance from the pri- 

 mary so as not to be influenced by it directly, but forming 

 with the secondary a single closed circuit, not only was the 

 distant coil capable of producing in an insulated wire helix 

 placed over it, a distinct current of induction at the interruption 

 of the primary, but sensible shocks were obtained from it. The 

 experiment was pushed still further ; and inductive currents of 

 a fourth degree were obtained. " By a similar but more ex- 

 tended arrangement, shocks were received from currents of a 

 fourth, and a fifth order: and with a more powerful primary cur- 

 rent, and additional coils, a still greater number of successive 

 inductions might be obtained. ... It was found that with the 

 small battery a shock could be given from the current of the 

 third order to twenty-five persons joining hands ; also shocks per- 

 ceptible in the arms were obtained from a current of the fifth 

 order." As Henry simply remarks : " The induction of currents 

 of different orders, of sufficient intensity to give shocks, could 

 scarcely have been anticipated from our previous knowledge of 

 the subject." By means of the small magnetizing helix intro- 

 duced into each circuit, the direction of these successive currents 

 was found to be alternating or reversed to each other. 



The concluding section of this important memoir is occupied 

 with an account of " The production of induced currents of the 

 different orders from ordinary electricity." An open glass cylin- 

 der about six inches in diameter was provided with two long nar- 

 row strips of tin foil pasted around it in corresponding helical 

 courses, the one on the outside find the other on the inside, 

 directly opposite to each other. The inner coiled strip had its 

 extremities connected with insulated wires which formed a circuit 

 outside the cylinder, and included a small magnetizing helix. 

 The outer tin foil strip was nlso connected with wires so that an 

 electrical discharge from a half-gallon Leyden jar could be passed 

 through it. The magnetization of a small needle indicated an 

 induced current through the inner tin-foil ribbon corresponding 

 in direction with the outer current from the jar.* By means of 



* About a year later, the distingnishpd German electrician Peter Riess, 

 apparently unaware of Henry's researches, discovered the secondary cur- 



38 



