156 ANNUAL OF SCIENTIFIC DISCOVERY. 



entire surface of each of the metallic plates must act. But the condenser 

 for every coil should be constructed in such a way that a small, or a con- 

 siderable part, or the whole of the surface of each plate may be applied to 

 the coil. For a large condenser which would make the effect of a coil ex- 

 cited by a single cell less than it would be without a condenser, will increase 

 the effect of the same coil when it is connected with a battery of ten or 

 twelve cells. 



ON A MODIFIED FOBM OF EUHMKOEFF'S INDUCTION APPARATUS. 



Mr. E. S. Ritchie of Boston, communicates to Silliman's Journal the fol- 

 lowing description of a modified form of RuhmkorfFs induction apparatus 

 of his own device. He says The induction apparatus made by Ruhm- 

 korff, is generally familiar. By it is obtained a spark of three-fourths of an 

 inch through the atmosphere. Mr. Hoarder has described in the London 

 Philosophical Magazine, (November and December 1846, ) certain improve- 

 ments by which he has lengthened the spark to three inches. The great dif- 

 ficulty experienced by him was in obtaining sufficient insulation between one 

 stratum of the wire and the next above or below it, the entire thickness of 

 the helix including wire and insulation bti.ig 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. Stohrer has adopted the plan of 

 dividing the coil into three divisions, thus lessening the difficulty ; still, 

 great danger exists of the spark passing which would ruin the helix. I have 

 endeavored to obviate this by winding the coil the entire thickness as it pro- 

 gresses. 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 fifty degrees. The diameter at the tube Avas about two and one half 

 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 cov- 

 ered 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, with less than 7,000 feet of wire, 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 purpose, I have obtained sparks over six inches long. I have con- 

 structed 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. Bentley's plan, the best. The surfaces used in the 

 instruments above described arc respectively about thirty and seventy-five 

 square feet. I have used all the interrupters alluded to by the writers above 

 mentioned, but prefer one which I have made thus : The anvil is a wire or 

 small rod of platinum secured in a plate by a binding-screw ; over this a 



