266 BULLETIN OF THE 



parallel with the former for about 35 feet and extended to form 

 an independent circuit (its ends being connected with a small 

 magnetizing helix,) was tested at varying distances beginning 

 with a few inches until they were twelve feet apart : a,t which dis- 

 tance of the parallel wire, its induction though enfeebled, still 

 indicated by its magnetizing power, a direction corresponding 

 with the primary current. The form of the room did not permit 

 a convenient separation of the two circuits to a greater dis- 

 tance.* 



The eminent French electrician Becquerel, in a chapter on 

 Induction in his large work, remarks : " Quite recently M. Henry, 

 Professor of Natural Philosophy in New Jersey, has extended 

 the domain of this branch of physics : the results obtained by 

 hira are of such importance, particularly in regard to the intensity 

 of the effects produced, that it is proper to expound them here 

 with some detail." Twenty pages are then devoted to these 

 researches, f 



A memoir was read before the Society, June 19th, 1840, giving 

 an account of observations on the two forms of induction occur- 

 ring on the making and on the breaking of the pi'imary galvanic 

 circuit, the two differing in character as well as in direction. In 

 these experiments he employed a Daniell's constant battery of 30 

 elements; the battery being "sometimes used as a single series 

 with all its elements placed consecutively, and at others in two 

 or three series, arranged collaterally, so as to vary the quantity 

 and intensity of the electricity as the occasion might require." 

 As the initial induction had always been found so feeble as to 

 be scarcely perceptible, (although in quantity sufficient to affect 

 the ordinary galvanometer as much as the terminal induction,) 

 most of the results previously obtained (such as the detection of 

 successive orders of currents) were derived from the strong in- 

 ductions at the moment of breaking the circuit. It became 

 therefore important to endeavor to intensify the initial induction 

 for its more especial examination : and this it was found could 

 be effected in two ways, — by increasing the "intensity" of the 

 battery, and by diminishing within certain limits the length of 

 the primary coil. 



" With the current from one element, the shock at breaking 

 the circuit was quite severe, but at making the same it was very 

 feeble, and could be perceived in the fingers only or through the 



* Tratis. Am. Phil. Soc. vol. vi. new series, art. ix. pp. 303-337. In 

 the Troceedings of tlie Society for November 2d, 1838, when this memoir 

 was read, it is recorded "Professor H^nry made a verbal comnmnieation 

 during the course of which he illustrated experimentally the phenomena 

 developed in his paper." (Proceed. Am. Phil. Soc. Nov. 2, 1838, vol. i. 

 p. 54.) ^ , 



t Traite experimental deVJ^hctriciie etduMagnetisme, vol. v. pp. 87-107. 



40 



