Mr. F. Guthrie on a Spiral Ley den Jar. 447 



blunt point to the first bend. A fine hole is drilled from the 

 centre of the blunt point to the bend. In this hole a needle 

 slides ; there is a button on the top to press it down by, and a 

 spiral spring to raise it again after being pressed down. In the 

 experiments j3 was moved about till there was no deflection ; the 

 button was then pressed and a dot made, ft was then removed 

 to another point, and the process was repeated until a sufficient 

 number of points for one line had been obtained; a was then 

 moved, and another line traced in the same manner. 



LIX. Note on a Spiral Ley den Jar. 

 By Frederick Guthrie*. 



A STRIP of tinfoil 4 feet long and 8 inches wide is placed 

 upon a strip of vulcanized caoutchouc 4 feet long and 

 1 foot wide, in such a way that along both sides there is a mar- 

 gin of two inches of caoutchouc, at one end (say the right) a 

 margin of four inches of caoutchouc, and at the left a margin of 

 four inches of tinfoil. A second piece of caoutchouc exactly 

 similar to the first is placed exactly over the first upon the foil. 

 A second piece of tinfoil of the same width as the first, but four 

 inches shorter, is placed on the second caoutchouc above the 

 first foil, with its right-hand end above the right end of the first 

 foil. Its left end, of course, falls four inches short. A brass 

 wire carrying a knob is laid across the end of the upper foil. 

 The whole is rolled up from the right end and bound. What 

 was the lower of the two foils projects between the two layers of 

 caoutchouc, and may be prolonged around the circumference of 

 the roll. It forms the outer coating or earth-surface. What 

 was the upper coating of foil now forms what corresponds to the 

 inner coating of the ordinary jar, and is entirely covered, except- 

 ing where it is prolonged as the wire and knob at the centre of 

 the roll. If the sheet caoutchouc be an eighth of an inch in 

 thickness, a jar of very great electrical capacity is obtained in a 

 very compact form, and one which is free from the risk of frac- 

 ture, and less impaired than the ordinary jar by atmospheric 

 moisture. 



A very serviceable modification of this form of jar has been 

 constructed for me by Mr.W. Peters f. The insulating material is 

 sheet ebonite. The construction is similar to the above. The 

 ends of the spiral roll are capped with dry mahogany disks. 

 The earth-foil is connected with a brass girdle around the centre 

 of the cylinder, and does not itself appear on the outside. The 

 electric capacity is between four and five times as great as that 

 of a glass jar of the same volume. It has been in use for several 

 months, and appears almost incapable of receiving injury. 



* Communicated by the Author. 



t W. Peters, 36 Whiskin Street, Clerkenwell, E.G. 



