THE MAGNETIC CIRCUIT ELECTROMAGNETS 41 



Up to 500 volts 0.045 in. 



For 1,000 volts 0.060 in. 



For 2,000 volts 0.080 in. 



For 3,000 volts 0.10 in. 



For higher pressures, up to 12,000 volts, add 0.03 in. per 1,000 

 volts increase. 



10. Calculation of Magnet Windings. The calculation of the 

 ampere-turns necessary to produce a given flux of magnetism 

 has already been explained (see Arts. 3 and 4), and it is a fairly 

 simple matter to determine the 

 exciting force approximately, 

 provided the magnetic circuit 

 consists mainly of iron of known 

 magnetic characteristics, and 

 that the air gaps are short. 

 These calculations will be more 

 fully illustrated when working 

 out one or two numerical exam- 



FIG. 16. Cylindrical magnet coil, 

 pies; but for the present it is 



assumed that a definite number of ampere-turns, SI, have to be 

 wound on a bobbin or former, and that the applied D.C. potential 

 difference, E, is known. 



If SI = the total ampere-turns in the coil shown in Fig. 16, 

 then, whatever may be the number of the turns S, the total 

 ampere- wires in the cross-section t X Hs (SI). For a first ap- 

 proximation of the area required, it is well to assume a certain 

 current density in the windings, which is not likely to cause an 

 excessive heat loss and therefore an unsafe rise of temperature. 

 The following figures may be used : 



For large magnets try A = 700, or (M) = 1,800 



For medium-sized magnets try A = 900, or (M) = 1,400 



For small magnets try A = 1,100, or (M) = 1,150 



where A = current density in amperes per square inch, and (M) 

 = number of circular mils per ampere. The relation between 

 these quantities, as previously explained (Art. 9) is, 



A X (M) = 1.273 X 10 6 



By assuming the current density, it is then easy to calculate the 

 probable cross-section of the copper in the coil. This, however, 

 is not equal to the product t X I because the winding space factor 



111 



13 S 



