202 



LORD JtAYLEIGH. 



the secondary capacity. For 3 cannot exceed a value estimated at about 

 1700 c.g.s.j whatever may be the magnetizing force of the primary 

 current. Thus if v be the volume of the core, the maximum energy *) is 



ÏNXvX 1700 2 ; 



aud the limit to V is found by equating this to \c[ F 2 , so tliat 



F = 1700 \/(Nvlq) (6) 



I have made a rough application of this formula to a coil in my 

 possession, with results that may be hère recorded. The core had a 

 diameter of 3 cm. and a leugth of 27 cm., so that v = 180 c. c. 

 From (5), properly applicable only to an ellipsoid, by setting2&=3, 

 2c = 27, we get N= *30. 



The capacity of the secondary is more difficult to deal with. In 

 modem coils the greater part would appear to arise from the positive 

 and négative potentials at the ends of the coil as opposed to the zéro 

 potential of the primari/ wire. The capacity between the primary and 

 secondary wires, considered as pôles of a condenser, can be calculated 

 and in many cases determined experimentally. The axial dimension of 

 the secondary of the coil above referred to is about 18 cm., and the 

 external diameter of the primary wire is about 5 cm., making the area 

 of each of the opposed surfaces 270 sq. cm. The interval between the 

 primary and secondary wire is '25 cm.; so that, taking the spécifie 

 inductive capacity of the intervening layer at 3, we get for the capacity 

 in electrostatic measure of the condenser so constituted 



1 2,70 



_XSX^=258c m . 2 ) -M 



Ouly a fraction of this, however, is operative in the présent case. On 

 the supposition of a coil constructed in numerous sections, the potential 

 in the middle will be zéro, the same as that of the primary wire, and 



x ) The energy of the primary current without a core is here neglected. 



2 ) Another coil by Apps, in which the insulation was sufficiently good to 

 allow the application of electrostatic methods, was tested experimentally. The 

 capacity between primary and secondary wires was thus found to be 120 cm., 

 less tlian the half of that calculated for the first coil. But in this case an 

 ebonite tube separated the two wires. 



