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XIV. On the determination of the size of the Wire which, wound 

 upon a Galvanometer or Electromagnet, will produce the maxi- 

 mum magnetic effect in a circuit of given external resistance, the 

 space taken up by the silk or other substance insulating the differ- 

 ent convolutions from each other being taken into consideration, 

 By G.K. Winter, Telegraph Engineer, Madras Railway Com- 



pany*. 



[With a Plate.] 



IT is easily proved, and generally known, that if it were not 

 necessary to insulate the different convolutions from each 

 other, or if the ratio between the space taken up by the insula- 

 ting covering and that occupied by the wire itself were constant 

 for all sizes of wire, the maximum magnetic effect would be pro- 

 duced when the resistance of the galvanometer or electromagnet 

 coil is equal to the sum of the external resistances in the circuit. 

 It is evident, however, that the ratio above mentioned varies with 

 the size of the wire; the thickness of the silk covering being the 

 same, or nearly the same, for all sizes of wire. 



Mr. Schwendler, in a paper in the Philosophical Magazine for 

 January 1867, taking the silk covering into account, proved that 

 the resistance of the coil should be less than the external resist- 

 ance, and gave formulae for determining what the coil resistance 

 should be in order to produce the maximum effect. 



It is my purpose in the following investigation, not to calcu- 

 late directly what the resistance of the coil should be, but to 

 determine the size of the wire when the magnetic effect is a 

 maximum; and I trust the result will prove of practical value 

 to electricians. 



In winding a magnet or galvanometer coil, the wire may 

 either be wound in layers alternately backwards and forwards, or 

 it may be wound in superposed layers^ the end of each layer 

 being connected with the commencement of the next; or if 

 greater insulation be required, eacb layer is sometimes separated 

 from the next above it by a layer of paper, gutta-percha tissue, 

 or other insulating substance. 



Plate II. figs. 1, 2, 3 represent sections of the coil with the 

 wire wound in the three methods above mentioned, and will 

 render what follows more easily understood. 



Space is undoubtedly best economized in the second of these 

 methods (fig. 2) f ; but, unfortunately, in order to coil the wire 

 in this way, all the layers must be wound in the same direction ; 



* Communicated by the Author. 



t This would be more evident if the size of the wire in the diagram were 

 not necessarily so exaggerated in proportion to the size of the coil. 



