Oscillatory Discharges. 31 



For the circle No. 2 : 



T =0-0000007, 

 L = 1768. 



In addition to these plane circuits we also made use of the 

 two following spirals in our period measurements. 



Spiral A. — This spiral is wound on an ebonite tube accu- 

 rately worked and traversed throughout its length by a glass 

 rod preventing any deformations of the former. The wire is 

 placed in a helix cut on the tube by means of the lathe. 

 The diameter of the copper wire it is made up of is 0"08 cm., 

 the mean radius of the cross-section of the spires being 

 0*713 cm. The number of spires per centimetre is 6*3025. 

 The whole of the spiral contains 485 spires. Its self-induction, 

 as measured by Nernst's method with currents of high fre- 

 quency (see p. 32), is 57,230 cms. 



Spiral B. — The support of this spiral is a large marble 

 cylinder, worked on the lathe with extreme care in the 

 mechanical workshop of our Institute. Its surface may 

 be considered as that of a practically perfect cylinder. The 

 variations of diameter of its cross-section, in fact (this being 

 23-821 cms. at 23° C), never reached 0'1 mm. 



The length of this cylinder is 98 cms. The spiral occupies 

 85 cms., the number of spires being 283. The mean thickness 

 of the wire it is made up of is 1*435 mm. The spiral itself 

 has been directly wound upon the marble by means of the 

 lathe. In order to prevent any displacement of the spires 

 the whole has been coated with a thick varnish layer of 

 gum-lac. The self-induction of this spiral is 4*546000 cms. 



28. To these values there have to be added the values 

 relative to the portions of the circuit establishing the communi- 

 cations between the condenser and the spark-gap. In order 

 to diminish the resistance and to have for those portions an 

 easily calculated self-induction, we established the connexions 

 by means of big brass tubes, their external radius being 

 p x = 1 cm., the internal radius p 2 = 0*88 cm., and of copper strips 

 5 cms. in width and -^ mm. in thickness. For these tubes 

 the self-induction is given by * 



I * Pi (Pl 2 -P2 2 ) 2 °>2 "A Pl-Rf i ' 



and for the strips by 



I being the length, a the thickness, and b the width. 

 * "SVien, Wied. Ann. liii. p. 928 (1894). 



