1 66 ELEMENTS OF ELECTRICAL ENGINEERING. 



When thermometers are applied to the free surface of a machine, it is desirable 

 that the bulb of the thermometer should be covered by a pad of definite area. A con- 

 venient pad may be formed of cotton waste in a shallow circular box about one and a 

 half inches in diameter, through a slot in the side of which the thermometer bulb 

 is inserted. An unduly large pad over the thermometer tends to interfere with the 

 natural liberation of heat from the surface to which the thermometer is applied. 



31. With apparatus in which the insulating materials have special heat-resisting 

 qualities, a higher temperature elevation is permissible. 



32. In apparatus intended for service in places of abnormally high temperature, a 

 lower temperature elevation should be specified. 



33. It is recommended that the following maximum values of temperature elevation 

 should not be exceeded : 



Commutating machines, rectifying machines and synchronous machines. 

 Field and armature, by resistance, 50 C 



Commutator and collector rings and brushes, by thermometer, 55 C. 

 Bearings and other parts of machine, by thermometer, 40 C. 

 Rotary induction apparatus : 



Electric circuits, 50 C., by resistance. 



Bearings and other parts of the machine, 40 C. , by thermometer. 

 In squirrel-cage or short-circuited armatures, 55 C., by thermometer, may be 

 allowed. 



Transformers for continuous service electric circuits by resistance, 50 C., other 

 parts by thermometer, 40 C., under conditions of normal ventilation. 



Reactors, induction and magneto-regulators electric circuits by resistance, 50 C., 

 other parts by thermometer, 40 C. 



Where a thermometer, applied to a coil or winding, indicates a higher temperature 

 elevation than that shown by resistance measurement, the thermometer indication 

 should be accepted. In using the thermometer, care should be taken so to protect its 

 bulb as to prevent radiation from it, and, at the same time, not to interfere seriously 

 with the normal radiation from the part to which it is applied. 



34. In the case of apparatus intended for intermittent service, except railway 

 motors, the temperature elevation which is attained at the end of the period corre- 

 sponding to the term of full load, should not exceed 50 C., by resistance in electric 

 circuits. In the case of transformers intended for intermittent service, or not opera- 

 ting continuously at full load, but continously in circuit, as in the ordinary case of 

 lighting transformers, the temperature elevation above the surrounding air-temperature 

 should not exceed 50 C. by resistance in electric circuits and 40 C. by thermometer 

 in other parts, after the period corresponding to the term of full load. In this 



where Rt is the initial resistance at room temperature t C. 



J?t+0 is the final resistance at temperature elevation 6 C. 



R Q is the inferred resistance at o C. 

 These combine into the formula 



6 = (238. 1 + /) (^ - i) degrees C. 



