THE MAGNETIC CIRCUIT ELECTROMAGNETS 45 



test resistances in the center of the coil when it is being wound. 

 The depth of winding has much to do with the relation between 

 outside and inside temperatures. This depth should rarely 

 exceed 3 in., and a long coil of small thickness will, obviously, 

 have a much more uniform temperature than a short thick coil 

 of the same number of turns. 



As a rough indication of what may be expected in the matter 

 of internal temperatures, it may be stated that, in magnet coils 

 of average size, the mean temperature might be 1.4 times, and 

 the maximum temperature 1.65 times, the external temperature. 

 The maximum allowable safe temperature for cotton-covered 

 wires is 95C., and as this may be reached when the outside tem- 

 perature is 40C. above that of the surrounding medium, a 

 maximum rise of temperature of 40 or 45C., as measured at 

 the hottest accessible part of the finished coil, is usually specified. 

 If the calculated temperature rise is in excess of this, the coil 

 must be re-designed in order to increase the cooling surface or 

 reduce the PR loss. 



The calculation of temperature rise is based largely upon 

 coefficients which are the result of tests, preferably conducted on 

 coils of the same type and size as the one considered. The cool- 

 ing surface of a magnet winding of the type shown in Fig. 16, page 

 41, may be taken as the outside cylindrical surface only; or this 

 outside surface plus the area of the two ends; or, again, the whole 

 surface, not omitting the inside portion in proximity to the iron 

 of the magnet core. This is largely a matter of individual choice 

 based on experience gained with similar types of coil, and the 

 heating coefficient will necessarily have a different value in 

 each case. 



The watts lost amount to PR, or El, or ^-. The heating 



coefficient is the cooling surface necessary to dissipate one watt 

 per degree difference of temperature between the outside of the 

 winding and the surrounding air. Thus 



TA. 

 ~ PR 

 and 



T = k (30) 



where T is the temperature rise in degrees Centigrade; k is the 

 heating coefficient, which can, if preferred, be properly defined 



