CONSTRUCTION OF COILS. 497 



CHAPTER VI. 

 CONSTANTS OF COILS. 



1086. CONSTRUCTION OF COILS. We have seen, in Chapters 

 IV. and V. of the First Part, how the properties of cylindrical and 

 circularly-wound coils may, apart from the resistance, be deduced 

 from the geometrical data of the coil alone ; but these calculations 

 presuppose a regularity of winding which is very difficult to obtain, 

 and which is only realised under exceptional circumstances. The 

 most minute precautions are necessary, for instance, that the radius 

 of each winding of the same layer be the same, and that there be 

 the same distance between each of them. 



The first condition is that the core is truly cylindrical ; this may 

 be ascertained by means of calliper compasses, or by a suitable 

 gauge. The frame is usually wood or metal. Metal frames may 

 be the seat of induction currents, which are very detrimental in 

 experiments where variable currents are made use of. The effects 

 of induction are attenuated by making a slit along a diametral plane, 

 which is filled up with an insulating plate of ivory or of ebonite. 

 Wooden frames are apt to get out of shape in the course of time ; 

 but if a hard dry wood be used, such as mahogany soaked with hot 

 paraffine particularly if care be taken to construct the frame from 

 a block consisting of a great number of pieces glued together, and 

 with fibres in different directions frames are obtained in which the 

 changes of shape may be altogether neglected. 



If the diameter of the wire, with its insulating envelope, is 

 constant, careful winding readily gives equidistant spirals. The 

 successive layers are usually separated by an insulating plate, such 

 as one or more sheets of paraffine paper. The use of a counter 

 enables us to ascertain the number of windings in each layer during 

 the coiling, and it is advisable to verify this number directly before 

 the layer is covered. The inclinations of the windings of successive 

 layers are obviously in opposite directions ; the effect of inclination 

 is neutralised by making the coil of an even number of layers. 

 VOL. n. K K 



