CHAP. XIII] TORQUE AND TRACTIVE EFFORT 



245 



the longitudinal tension, and both are numerically equal to the 

 density of the stored energy. 



As an application of the lateral action, consider a constant- 

 current or floating-coil transformer (Fig. 59), used in series 

 arc-lighting. The leakage flux is similar in its character to that 

 shown in Figs. 50 and 51. The lateral pressure of the leakage 

 lines between the coils tends to separate them, acting against 

 the weight of the floating coil. A part of this weight has to be 

 balanced by a counter-weight Q because the electro-magnetic 

 forces under normal operation are comparatively small. 



Since the currents are alternating, the force is pulsating, 



X3of 



Primary CM* tata*ll 



FIG. 59. A floating-coil constant-current transformer. 



but is always in the same direction, tending to separate the coils. 

 The average force depends upon the average value of H' 2 , in 

 other words, upon the effective value of the current. According 

 to eqs. (170) and (172), we have 



win TO S is the area of the floating coil in contact with the flux. 

 With the assumed paths for the lines of force, and neglecting 

 the reluctance of the iron core, we have that 



#,//-n/ f ///1000f kiloamp.-turns/cm., . . (174) 



where / i- the leiiiTtli of the lines of force in the air. in cm., and 



Ul fff is the in. in. f. nf either coil. Tile force of repulsion 18 prO- 



portional to the square of the current, and is independent of the 



