THE MAGNETIC CIRCUITELECTROMAGNETS 39 



many cases enamelled wire protected by a single covering of 

 cotton has been used with very satisfactory results. In all cases 

 when it is desired to save space by reducing the thickness of 

 insulation on wires, the points to be considered are: (1) insulation; 

 (2) durability; and (3) cost. The price of the silk covering is of 

 course much higher than that of the cotton covering. 



Enamel insulation does not add much to the diameter of the 

 wire as will be seen by reference to the accompanying table based 

 on data kindly furnished by the Belden Manufacturing Co. of 

 Chicago. This wire will not suffer injury with the temperature 

 maintained- at 200F. continuously, and it will withstand without 

 breakdown a pressure of 900 volts per mil thickness of enamel; 

 but on account of the possibility of abrasion during winding, a 

 large factor of safety (not less than four) should be used, and 

 indeed it is always advisable to place paper between the layers of 

 enamelled wire, unless a careful study of the conditions appears 

 to justify its omission. 



Triple cotton covering can be used with advantage on the 

 larger sizes of wire when the working pressure between adjacent 

 turns exceeds 20 volts. When extra insulation is required be- 

 tween the layers of the winding, this is usually provided in the 

 form of one or more thicknesses of paper or varnished cloth. 

 It is the insulation between the finishing turns of a layer of wire 

 and the winding immediately below which requires special at- 

 tention, because this is where the difference of potential is great- 

 est. One advantage of the ordinary cotton covering is that it 

 lends itself admirably to treatment with oil or varnish, either 

 before or after winding. 



Space Factor. The amount of space taken up by the insulation 

 and the air pockets between wires of circular cross-section is 

 important, because it reduces the cross-section of copper in the 

 coil. If A is the cross-section of the copper, and A' the total area 



of cross-section through the winding, the ratio -j-, is called the 



space factor. The calculated space factor, based on the assump- 

 tion of a known diameter over the insulation, and a close packing 

 of the wires, does not always agree with the value obtained in 

 practice, but the curves of Fig. 15 will be found to give good 

 average values. It will be understood that the space factors of 

 Fig. 15 include no allowance for extra insulation between layers 

 of wire or for the necessary lining of the spool upon which the coil 



