162 J- K. Roberts: 



(iv.) The length of the armature nuist not exceed 13 cm. 

 (v.) Eough calculations shewed that the thickness of the copper- 

 should be 3 or 4 mm. 

 The copper was made 3.3 mm. thick, and 5.45 cm. in external 

 diameter. 



This makes the electrical resistance from end to end 4.46x10^ 

 E.M. unit. 



From eciuation (4) we multiply this by 20 to obtain the value of 

 p = 20x 4.46x10^^ 



Calculating the magnetic resistance from the dimensions we obtain. 

 the value 



Magnetic resistance = 0.040. 



Since the air gap is 1.9 cm. long it can be inferred from the 

 experiments carried out that this will lio the effective value of the- 

 magnetic resistance. 



The flux obtainable is therefore given by 



</, = .47r('ui)/0.04 = 107r(m) 



whei'e ni is the number of ampere turns. 



It is proposed to run the apparatus at 1500 revolutions per' 

 mintite, i.e., N — 25. 



Power = 27rNi// 



= 277-N7)0-(/;- + 47r-N'''/V-^) " ' 



Since i-Horse Power is required we have 



3.73 X 10" = 1-086 X 10'"-(7ti)"(7.95 x 10" + 2.47 x 10^\-)r.'...(6) 

 If /\=r92 and 382 as in Equation (5) we obtain respectively 

 7a = 5.38 X 10'^ and 5.86 x 10'' (7) 



The inductance term should not be greater than 356, as the air 

 gap is nearly ten times as large as in the Experiment (b). 



The heat generated in a winding of given size depends on the 

 number of ampere ttirns. 



It is, therefore, necessary to discover whether it is possible to dis- 

 sipate the energy which would be generated in a winding of the 

 size required by the dimensions of the apparatus. In making cal- 

 culations it can be safely assumed that .05 watt can be dissipated 

 per square cm. of the area of winding. With this assumption the- 

 possible number of ampere turns on each coil comes out to be 4500 

 — that is, a total of 9000, which 'is more than is required. As only 

 a limited voltage is available it is necessary to choose wire of such 

 a gauge that the resistance will be low enough to allow the requisite- 

 number of ampere turns. 



The poles were wound with 4720 turns of 24 gauge copper wire-. 



