74 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
The striking distance is very little more for 8000 than eight times (8 ’487) that 
given for 1000, which may be too great for the reason stated in page 85. 
On the suggestion of Professor E. Mascart, of the College de France, Paris, who 
honoured us with a visit to our laboratory in April, 1877, we have determined the 
striking distance between two concentric cylinders ; fig. 1 1 shows the arrangement of 
the apparatus. S S, S' S' are two ebonite stands supporting the outer cylinder A A of 
brass, 2\5 inches long, and bored with a perfectly true hole, 0’4895 inch diameter ; B 
is the inner cylinder, also of brass, which was turned at first nearly of the diameter of 
the hole of the outer cylinder ; on its axis, of one piece with the cylinder, are fitted 
two ebonite cylinders E E, exactly fitting the hole ; their outer ends have a groove 
Fig. 11. 
turned in them, in which the string of a hand-bow works to turn the cylinder on its 
centres c, c when it is wished to reduce it in size ; C, C' are clamps for connecting 
the outer and inner cylinder respectively with the battery. When it is wished to 
make the inner cylinder less in size, the clamp C is removed, and the cylinder is put 
on dead-centres, and made to rotate with the hand-bow, and reduced by turning off a 
very small quantity at a time between each trial of the striking’ distance ; the quantity 
removed between each trial was generally 0’001 inch or less, and by this means the 
striking distance was ascertained to 0’0005 inch. 
In the first instance the same cylinder was reduced to determine the limit of the 
striking distance for the several numbers of cells employed ; on a repetition of the 
experiment, seven inner cylinders were turned to the size at which the spark just 
passed and retained ; this delicate work was executed with great care and skill by 
Mr. Fram. 
Table VIII. — Length of spark between two concentric cylinders. 
Curve. 1200 cells. 
2400 cel's. 
3600 cells. 
4800 cells. 
5880 cells. 
6960 cells. 
8040 cells. 
Date. 
inch. 
inch. 
inch. 
inch. 
inch. 
inch. 
inch. 
0-0086 
0 0248 
0-0534 
0-0655 
0-0780 
0-0897 
0-0983 
June 5, 1877. 
0-0093 
0-0281 
0-0451 
0-0638 
0-0711 
0-0821 
0-1015 
June 29, 1877. 
XT. Mean. 0-0089 
0-0264 
0-0492 
0-0646 
0-0746 
0-0859 
0-0998 
Plate 7. 
In taking the striking distances the current was reversed after a first trial, so as to 
