Dr. Fleming on the Cunioineter. 759 



by the employment of a vacuum-tube of the spectroscopic 

 type preferably containing rarefied Neon, which is connected 

 to the plates of the condenser. 



Such an arrangement constitutes the instrument called by 

 the author a cymometer*. 



In its practical and most recent form it is constructed as 

 follows: — On a mahogany baseboard about 136 centims. long 

 and 30 wide is mounted an inductance-coil H, H (see Plate XL) . 

 This consists of an ebonite tube 108 centims. long and '6 cms. 

 diameter, having a helical groove cut on its surface, with 

 4 turns to the centimetre (10 to the inch exactly). In this 

 groove is wound a bare copper wire 1*22 mm. in diameter, 

 the ends of which are secured to brass collars clamped on the 

 ebonite tube. The inductance L of such a spiral can be 

 calculated very approximately by the formula 



L = (ttDX) 2 /, 



where D is the mean diameter of one circular turn of the 

 helix and X the number of turns of wire per centimetre or 

 otherwise. 7rDX represents the length of wire which is wound 

 on one unit of length (1 cm.) of the helix, and / is the length 

 of the helix. 



This helix is supported by wood brackets, a, b, on the 

 base-board. 



Parallel to it is fixed a sliding- tubular condenser. This 

 consists of a brass tube I, having a solid pin fixed in one end 

 by means of which it is clamped in ebonite clips fixed to a 

 wooden block c. This tube has a length of 104 centims. and 

 an outside diameter of 2*49 centims. Over this brass tube 

 is fixed tightly a-thin ebonite tube E, the sides of which are 

 only 1*6 mm. thick, and this extends for a length of 5 centims. 

 beyond the inner brass tube. Over this ebonite tube slides 

 easily another brass tube 0, 100 centims. long and 305 

 centims. outside diameter. This tube has a heavy collar, k, 

 at one end. to which is attached an ebonite handle. //. by 

 which to displace the tube, and also a projecting pin. /, which 

 carries a semicircular collar, K. resting on the inductance 

 spiral HH. Also this pin or rod carries an index P, which 

 moves over a divided scale ^SS. 



The instrument is completed by a copper bar, Lj L 2 L 3 , of 

 square section, 6 mm. in side, which joins one end of the 

 induct mce spiral to the inner tube of the condenser. The 

 outer tube of the condenser moves through an insulated 

 metal ring, rj, and from this ring and from a terminal T at 



• I". oni K\\m. -,\ wave. The autlmr b indebted tc hie colleague Pro£ 

 Piatt, .M.A., for the correct form of this word. 



