THE MAGNETIC CIRCUIT ELECTROMAGNETS 33 



lowing to the only other materials of consequence in the design of 

 magnets or dynamos, namely, the copper wire, which is the 

 material universally used for the windings, and the insulating 

 materials, which prevent electrical contact between neighboring 

 turns of wire, and also between the winding as a whole and the 

 iron of the magnetic circuit or supporting framework. 



For the operation of electromagnets, high voltages are rarely 

 used, and the provision of appropriate insulation presents no 

 serious difficulties; but it must not be overlooked that, when 

 the inductance is great i.e., when the flux links with a large 

 number of turns and the product maxwells X number of turns 

 is large there may be, at the instant of switching off the current, 

 differences of potential between neighboring turns of wire, 

 considerably in excess of the normal potential difference cal- 

 culated on the assumption of a steady impressed voltage between 

 the terminals of the coil. In the design of continuous-current 

 machines, pressures up to 5,000 volts may have to be considered, 

 and in alternating-current generators, the pressure may be as high 

 as, but rarely in excess of, 16,000 volts. The higher pressures, 

 as used for transmission of energy to great distances, are obtained 

 by means of static transformers, and the question of insulation 

 then becomes of such great importance that it has to be very 

 thoroughly studied by experts. Pressures of 100,000 volts are 

 now common for step-up transformers, and there are many 

 transformers actually in operation at 150,000 volts and even 

 higher pressures; so that the provision of the requisite insulation 

 for machines working at pressures not exceeding 16,000 volts 

 (which is the limit for any of the designs dealt with in this book) 

 offers no insuperable difficulties. It is, therefore, proposed to 

 devote but little space to the discussion of insulation problems; 

 although, as occasion arises, data and information of a practical 

 nature will be given. 



Copper WireWith silver as the one exception, copper is 

 the metal with the highest electrical conductivity; it is also 

 mechanically strong, easy to handle, and generally the most 

 suitable material for electrical windings. The resistance of a 

 given size and length of wire is usually obtained by reference 

 to a wire table, similar to the accompanying tables, which con- 

 tain such information as the designer of electrical apparatus 

 requires. The very large, and the very small, sizes of wire are 

 omitted; but wire tables for the use of electrical engineers are 



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