248 MEMORIAL OF JOSEPH HENRY. 



a Leyden jar. From various experiments, the limit of efficient 

 length for a given galvanic power was ascertained ; beyond which 

 the induced current was diminished. Employing a Cruickshanks 

 battery of 60 small elements (4 inches square) he found with the 

 ribbon coil that the induced currents were exceedingly feeble, but 

 with the long wire helix as the primary circuit that strong indica- 

 tions were produced. By the alternations of the ribbon and wire 

 coils, the fact was established " that an intensity current can induce 

 one of quantity, and by the preceding experiments the converse has 

 also been shown that a quantity current can induce one of intensity;" 

 a result which has had an important bearing on the subsequent 

 development of the electro-magnetic "Induction-Coil." With a 

 long ribbon coil receiving the galvanic current from 35 feet of zinc 

 surface, sensible induction shocks could be felt from a large annular 

 coil of four feet diameter (containing five miles of wire) when placed 

 in parallelism at a distance of four feet from the primary coil ; while 

 at the distance of one foot the shock became too severe to be taken. 

 With this arrangement an induction shock was given from one 

 apartment to another, through the intervening partition. 



Successive orders of Induction. When it is considered that the 

 primary current in such cases has a considerable duration, while 

 the secondary current is but momentary, being developed only at 

 the instant of change in the primary, it could certainly not have 

 been expected that this single instantaneous electrical impulse of 

 reaction would be capable of acting as a primary current, and of 

 similarly inducing an action on a third independent circuit: and 

 during the seven years in which galvanic induction had been known, 

 no physicist ever thought of making the trial. Theoretically it 

 might perhaps have been inferred, if such tertiary induction 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 con- 



