Intelligence and Miscellaneous Articles. 239 
England. I have no desire to press this discussion further; but 
if I find it at all necessary, I shall not hesitate to publish the whole 
of the correspondence betwixt Dr. Noad and myself, by which my 
position with regard to him and Mr. Bentley will be still better 
understood. In conclusion, I may state that the whole of this very 
unpleasant affair would have been avoided if Dr. Noad, when he 
introduced the machine to the public, had been candid enough to 
allude to my previous labours, which he himself was the means of 
keeping from the public. 
I have the honour to remain, Gentlemen, 
Yours very obediently, 
28 Buckwell Street, Plymouth, JonaTHAN N. Hearper. 
August 10, 1857. 
ON A MODIFIED FORM OF RUHMKORFYF’S INDUCTION APPARATUS, 
BY E. 8. RITCHIE. 
The induction apparatus made by Ruhmkorff and described by 
Du Moncel is probably familiar to many of your readers. By it is 
obtained a spark three-fourths of an inch through the atmosphere. 
Mr. Hearder has described in the London Philosophical Magazine 
(Noy. and Dec. 1856) certain improvements by which he has length- 
ened the spark to 3 inches. 
The great difficulty 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 insu- 
lation—being only about half an inch, and a tension of electricity 
sufficient to throw a spark 3 inches existing between the outer and 
inner strata. 
M. 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 endeavoured to obviate this by winding the coil the entire 
thickness as it progresses. 1 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 24 inches, and the greatest diameter 34 inches, the 
length of the cone being nearly 4 an inch. When the stratum was 
laid and cemented by resin and bees-wax, a ring of thin vulcanized 
india-rubber was stretched over and cemented, the wire passed down 
to the glass cylinder, and this wire covered also by india-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 7000 feet of wire, I ob- 
tained a spark of 24 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 6 inches 
long. 
I have constructed the condenser with oiled silk, with very thin 
