ApEiL 33, 188?. J 



sciJEJsrc^. 



399 



of Henry's work ran parallel with that of the 

 most distinguished experimental physicist of this 

 or, indeed, any age. In several instances they 

 attacked the same problem almost simultaneously, 

 and often independently of each other. The great 

 discovery by Faraday, on Sept. 24, 1831, of electro- 

 magnetic induction, inaugurated an era of greatly 

 increased activity in electrical research . Henry had 

 thought much concerning the relation of magnetism 

 to electricity, and had devoted the early part of 

 the same year to his very important research look- 

 ing to the improvement of the electro-magnet, 

 with the intention of making use of it in an attack 

 upon the then unsolved problem. The pressure 

 of other duties prevented him from taking up 

 the work until after the commencement of Fara- 

 day's success, but his improvement of the magnet 

 was of sufficient importance to stand alone as a 

 most valuable contribution, since through it Morse's 

 system of telegraphy was made possible. 



He at once repeated Faraday's experiments, and 

 extended them, with interesting results. The 

 difficulties under which he labored, arising out of 

 his occupation, and also from the difference, far 

 greater then than now, between London and Al- 

 bany or Princeton as centres of intellectual activity, 

 were more considerable than those which his dis- 

 tinguished contemporary was obliged to overcome. 

 Those were the days in which quantitative meas- 

 urements in electricity were made by comparison 

 of sparks produced on file and rasp, by observing 

 rapidity of decomposition, by the magnetization 

 of sewing-needles, or in which men felt their way 

 to results through shocks in the arms, fingers, or 

 tongue. In those days batteries were inconstant 

 and short-lived, connections were made with 

 mercury cups, conductors were carefully insulated 

 by a silk covering put on by the experimenter 

 himself, and 'bell-wire' was almost the only 

 available material for circuits. Henry independ- 

 ently produced the spark from the magnet, but 

 afterwards learned that he had been anticipated in 

 the observation in England. In 1832 he discov- 

 ered self-induction in a long wire, and correctly, 

 though somewhat hesitatingly, interpreted the 

 phenomenon. This was not observed by Faraday 

 until 1834, and at first he did not comprehend the 

 true nature of the operation. He corrected his 

 error in 1835, and the credit of the discovery has 

 been generally accorded to him. At an early date, 

 Henry produced current-induction by means of 

 ' common ' electricity, which Faraday had not at 

 first been able to accomplish. 



In one of his numerous variations of Faraday's 

 experiment, in which he used flat coils or spirals, 

 he tried the effect of interposing a conducting- 

 plate between the primary and secondary coils. 



He found that the shock from the secondary coil 

 was almost totally destroyed by the introduction 

 of a plate of copper or other conducting- material 

 between it and the primary. 



This was an important conclusion, and led to 

 important results. Shortly after its publication, 

 he received from Faraday a copy of his fourteenth 

 series of experimental researches, in which he 

 makes a statement diametrically opposed to that 

 of Henry in reference to this effect, being, in sub- 

 stance, that the interposition of a conducting-plate 

 made not the slightest difference in the result. 

 This naturally excited in Henry a lively interest in 

 the question, and he made an extensive investi- 

 gation in order to determine which view was 

 erroneous. 



Curiously enough, both were correct. Faraday 

 used a galvanometer in his experiments : Henry 

 observed the strength of shocks, or the physiologi- 

 cal effect. There are undoubtedly induced cur- 

 rents in the interposed conductors ; but they will 

 be transient, and their integral effect on the num- 

 ber of lines of force passing through the secondary 

 will be zero. The effect, then, will be that the 

 time of the rise and fall of the induced current 

 will be altered. The variation taking place with- 

 in a small fraction of the period of the galvanom- 

 eter needle, the throw of the needle will not be 

 changed ; but the effect of the shock will be great- 

 ly modified, and may become insensible. Henry 

 did not leave this question until he thoroughly 

 understood the cause of the discrepancy. 



The most important result of his original ex- 

 periment, however, was that it led him to the 

 discovery of induced currents of the second, third, 

 and fourth orders. 



It is not possible to refer, in this place, to many 

 other investigations of great interest which are to 

 be found recorded in part i. A few of them relate 

 to other departments of physical science, and some 

 of them are not well known, even to his own coun- 

 trymen. On the very first page will be found an 

 account of a most admirable lecture experiment, 

 which might well find a place in our modern 

 courses, but which is probably not generally 

 known to professors of physics. 



Many lovers of pure science will find it hard, 

 after a perusal of part i., to avoid a feeling of regret 

 that Henry was not allowed to continue his re- 

 searches, instead of being called to the directorship 

 of the Smithsonian institution. That he was excep- 

 tionally well qualified for this important post, no 

 one will deny, although it must have been accepted 

 at a sacrifice which no one understood better than 

 Henry himself. Throughout his long connection 

 with the institution, and during a career which 

 needs no praise and requires no comment, he con- 



