296 LOCALIZED ELECTRIZATION. 



The coils of the armature are formed by a wire one-tenth of a 

 millimetre in diameter, and about 170 metres in length ; those 

 of the magnet are formed by 280 metres of the same wire. 



"M. Gaiffe considers that the coils of the armature furnish 

 about two-thirds of the effect, and those of the magnet one-third 

 only. 



" The currents of the armature change their direction twice 

 during one revolution of the axis on which they are carried, while 

 those of the coils rolled round the magnet change four times, — the 

 maxima of the two first being considered as coinciding with two 

 of the second ; and it may be said that, by an arrangement of the 

 wire, the directions of the currents are the same. As the currents 

 change their direction in the reels of the magnet before they 

 change in those of the armature, their effects are at one moment 

 in part destructive of each other, unless controlled by a com- 

 mutator arranged to correct them singly in proper time ; but this 

 addition would be an extreme and perfectly useless complication, 

 because the currents that are produced are utilized by an in- 

 terrupter of the kind that has been described with reference to the 

 machine of Clarke. 



" This interrupter acts only at the moment when the currents 

 of the armature are at their maximum ; because that is the time 

 at which is established the additional action of the two currents. 



" The apparatus also possesses a redresser of the current analo- 

 gous to that of Clarke, but with no other office than to give the 

 same direction to two discharges which are produced at each revo- 

 lution. This direction is indicated by marks on the binding-screws 

 which serve to give attachment to the conductors ; and the fur- 

 nishing of such marks is a care that cannot be too strongly recom- 

 mended to manufacturers. The direction of the discharges is not 

 a matter of indifference, and manufacturers should undertake to 

 furnish to practitioners every possible facility for determining it." ^ 



2.— Electro-Dynamic Instruments. 



A. — RuhmJcorf's Induction Coil. — " In the cases, although they 

 are somewhat rare, in which sparks of high tension are required, 

 there is no better source of them than the instrument which has 

 gained for M. Euhmkorff a well-merited celebrity. We give (fig. 

 78) an illustration of a small model, which produces sparks eight 

 or ten miia~»:\etres in length, and which can be borne when they 

 do not sucCt>3d one another too quickly. These coils of relatively 

 strong tension may replace the electricity of the ordinary friction 



2 Le Koux, he. eit.. p. 31. 



