E. S, Ritchie on a Modified Induction Apparatus. 45 



Art, IV. — On a Modified form of Ruhmhorff^s Liduction Appa- 



^ ratus ; by E. S. EiTCHiE, 



The Induction Apparatus inacle by Rulimlvorif and described 

 byDuMoncel is probably familiar to many of your readers. 

 By it is obtained a spark of tliree-fourtlis of an inch through 

 the atmosphere. Mr. Hearder has described in the London 

 Philosophical Magazine, (Nov. and Dec. 1846,) certain improve- 

 ments by which he has lengthened the spark to three inches. 



The great difficulty experienced by him was in obtaining suffi- 

 cient insulation between one stratum of the wire and the next 

 above or below it, the entire thickness of the helix — including 

 wire and insulation — ^being only about half an inch, and a tension 

 of electricity sufficient to throw a spark three inches existing 

 between the outer and inner strata. 



Mr, Stcihrer has adopted the plan of dividing the coil into 

 three divisions, thus lessening the difficulty : still, great danger 

 exists of the spark passing Avhich would ruin the helix. 



I have endeavored to obviate this by winding the coil the 

 entire thickness as it progresses. I commenced with a glass tube 

 or bobbin, laying the first course on a cone at as great an angle 

 as the wire could be conveniently laid — say about 50°. The 

 diameter at the tube was about two and one naif inches and the 

 greatest diameter three and one half inches, the length of the 

 cone being nearly half an inch. When the stratum was laid, 

 and cemented by resin and bees- wax, a ring of thin vulcanized 

 rubber was stretched over and cemented, the wire passed down 

 to the glass cylinder, and this wire covered also by rubber; then 

 another stratum was laid in the same manner; — that is, the coil 

 is built up precisely as a cop is laid by a mule-spinner. The 

 advantages are that the wire in each conical layer is very short, 

 and only a slight tension can exist between them. 



With a helix thus made, wdth less than 7,000 feet of w^ire, I 

 obtained a spark of two and one quarter inches; and with one 

 since constructed on the same principle, with 30,000 feet of 

 wire, differing only so far as I found necessary to enable me to 

 wind the helix by a machine which I constructed for the pur- 

 pose, I have obtained sparks over six inches long. 



I have constructed the condenser with oiled silk, with very 

 thin gutta percha, and with paper of different thicknesses; but 

 ■find tissue paper varnished and used double, according to Mr. 

 Bentlej^'s plan, the best. Tlie surfaces used in the instruments 

 above described are respectively about thirty and seventy-five 

 square feet. I liave used all the interuptors alluded to by the 

 writers above mentioned, but prefer one wdiich I have made thus : 

 The anvil is a wire or small rod of platinum secured in a plate 



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