CONTENTS ix 



PAGE 



CHAPTER XIII. THE MECHANICAL FORCE AND TORQUE DUE TO ELEC- 

 TROMAGNETIC ENERGY 240 



The density of energy in a magnetic field. The longitudinal 

 tension and the lateral compression in a magnetic field. The deter- 

 mination of the mechanical forces by means of the principle of virtual 

 displacements. The torque in generators and motors. 



APPENDIX I . . 262 



Ari'Kxnix II... . 266 



