162 J- A^ Roberts: 



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

 (v.) Rough calculations shewed that the thickness of the copper 

 should be 3 or 4 nun. 

 The copper "vvas 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 equation (4) we multiply this by 20 to obtain the value of 

 p = 20x4.46x 10^ 



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 be the effective value of the 

 magnetic resistance. 



The flux obtainable is therefore given by 



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



where m is the number of ampere turns. 



It is proposed to run the apparatus at 1500 revolutions per 

 minute, i.e., N— 25. 



Power = 27rNi/^ 



Since |-Horse Power is required we have 



3.73 X 10^=1-086 X W^(nif {7.9o x 10" + 2.47 x WX^jZ^.^Q) 

 If A. = 92 and 382 as in Equation (5) we obtain respectively 

 7U = 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 turns. 



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

 sipate the energy which w^ould 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 Avatt can be dissipated 

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

 possible number of ampere turns on each coil cemes 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. 



I 



