8 Lord Kelvin. On a new Form of Air Ley den, [June 2, 



FIG. 2. 



system A. The wnole instrument, as said above, rests on three 

 vulcanite legs attached to the base plate of system A ; and two 

 terminals are provided, one, i, on the base of system A, and the 

 other,;, on the end of one of the corner bolts of system B. 



The air leyden which has been thus described is used as a standard 



of electrostatic capacity. In the instrument actually exhibited to the 



Society there are twenty-two plates of the system B, twenty-three of 



the system A, and therefore forty-four octagonal air spaces between 



the two sets of plates. The thickness of each of these air spaces is 



approximately O301 of a centimetre. The side of each square is 



'13 cm., and therefore the area of each octagonal air space is 85'1 



sq. cm. The capacity of the whole leyden is therefore approximately 



< 85-l/(47rXO-30]), or 990 cm. in electrostatic measure ; or I'l x ICT 18 



c.g.s., electromagnetic measure; or 1-1 x KT 9 farads, or I'l x l()- 3 micro- 



This is only an approximate estimate founded on a not 



imute y accurate measurement of dimensions, and not corrected for 



e addition of capacity, due to the edges and projecting angles of 



the squares and the metal cover. I hope to have the capacity deter- 



l with great accuracy by comparison with Mr. Glazebrook's 



standards in Cambridge. 



To explain its use in connexion with an idiostatic electrometer for 



he direct mewurement of the capacity of any insulated conductor, 1 



1 suppose, for example, this insulated conductor to be the 



