376 PROCEEDINGS OF THE AMERICAN ACADEMY 



and De la Rive's experiment of the revolution of the lumi- 

 nous streams in vacuo around the poles of a magnet. 



Professor W. B. Rogers called attention to the nature and 

 extent of the improvements which Mr. Ritchie had made in 

 the Ruhmkorff apparatus. 



In the construction used by Ruhmkorff and others, the outer helix 

 is wound in strata or courses parallel to the axis of the coil, separated 

 from one another by layers of insulating material. This brings into 

 proximity parts of the circuit which are really remote from each other 

 as measured along the course of the wire, and by the unequal tension 

 of the electric wave at these points tends to produce discharges within 

 the helix or around its ends. 



In instruments having a moderate length of wire, the insulation is 

 sufficient to prevent such discharges ; but when the coil is made up of 

 a great number of these superimposed courses, the enormous difference 

 of tension between the outer and inner parts of the helix overcomes 

 -the resistance of the insulating material, and either destroys the action 

 of the instrument by an internal discharge, or wastes a great part of its 

 energy by frequent sparks around the extremities. If to obviate these 

 evils we increase the thickness of the insulating sheets, we augment in 

 the same proportion the distance through which the primary coil exerts 

 its inducing power upon the outer helix, and thereby in a still higher 

 ratio impair the energy of the induced curre'nt. Thus constructed, 

 therefore, it would appear that the power of the instrument is unavoid- 

 ably restricted within moderate limits. 



It is but just to state, that Professor Poggendorff was the first to 

 point out this defect in the Ruhmkorff apparatus, and to attempt its 

 remedy by dividing the helix into short sections. But he made no 

 further application of the principle than to construct instruments with 

 eight short coils placed end to end, which, although somewhat more 

 effective, were still, as he confessed, too much exposed to " the 

 disturbing effects of internal sparks " to present a very decided 

 advantage. 



Mr. Ritchie, abandoning the attempt to improve the apparatus on 

 the old construction, determined on huilding up the helix from a series 

 of thin strata or rings placed peipendicular to the axis. By this ar- 

 rangement the distance between the points of a stratum increasing with 



