ar — > '- will be called the magnetic air-resistance, a 



498 Profs. Ayrton and Perry on the 



0i, the value of j3 in the iron of the armature. 

 j3/, the greatest value of /3 which it is convenient to employ- 

 in the armature when the machine is giving its greatest 

 permanent output. 

 N, the total induction in the iron of the armature H$ = (3 l ai 



or & kr~L. 

 28 

 a 2 



not very incorrect or misleading expression. 



lv 

 — will be called the magnetic resistance of a portion of the 



iron part of the magnetic circuit whose length is I, 

 whose cross section is a, the permeability being fi, 

 Nv being the total induction there. 



The whole iron magnetic resistance mav be written % — 

 & " ^a/ju 



This is for only one of the magnetic circuits if the machine 



has more than one. 

 In a well-designed modern machine there is no throttling 



of the induction anywhere, and — may be taken as 



a lf M 



the whole magnetic iron resistance, /x being the per- 

 meability of the iron of the armature, I the average 

 total length of a line of induction in the iron of the 

 whole machine. 

 W watts, the power developed by the rotation of the 

 armature. 



"W 7 , the permanent highest power developed. 



EO, the rate of loss of heat in watts per square centim. of 

 the cylindric outer surface of the armature. This is 

 usually taken to be 0"2 in modern machines furnished 

 with some ventilating arrangement. The value of 



'FT? 

 — may, we find, be practically taken as 288 in 



such machines ; we shall call this constant q, as 

 machines made by different makers differ greatly in 

 its value, even where there are the same methods of 

 ventilating and the same sizes of wires. With finer 

 wires p is greater, as the space is less occupied by 

 copper, so that q is less. If two thirds of the space 

 be taken as occupied by copper, ^=288 if E#=0*2. 

 In some armatures, which are always perhaps too safe 

 from heating, we have found q to be as little as 150. 

 lu some cases it is considerably more than 300. 



V ] - 



