Possible Existence of Mutuat Induction between Masses. 593 

 its energy. We shall then receive back the E.M.F. time- 



integral J Edt. 



Similarly, if M is the moment of inertia of the flywheel, 

 A in fig*. 1, a rotational impulse 



Tdt=hl d ^dl = Mu 



JH 



dt 



will give to the flywheel an angular velocity, « (where T 

 stands for the turning moment necessary to give the angular 

 acceleration dec/dt). If there is no friction, the flywheel will 

 go on spinning, and it will contain a store of energy, -pla 2 . 

 The wheel can be stopped by applying an opposite turning 

 moment, and can be made to yield up its energy. We shall 

 then receive back the turning-moment time-integral ^Tdt. 

 So far the analogy is complete. But now observe the effect 

 of the electric circuit Ae upon the adjacent circuit B e . 



When di/dt has some value generating the E.M.F. E in 

 the circuit A e there exists also an E.M.F. in the circuit B e , 

 the amount of which will depend upon the closeness of the 

 magnetic coupling to the two circuits. Any change in the 

 current in A e will be accompanied by a tendency for 

 current to flow in B e . 



The question naturally arises, " Is there any action 

 between masses analogous to this mutual induction between 

 electric circuits ?" If we accelerate the flywheel A (fig. 1), 

 does it produce any force upon the suspended disk B ? 

 It is quite possible that a small force of the kind might pass 

 unnoticed if not specially looked for, just as the gravitational 

 attraction between two movable objects would ordinarily 

 escape observation. 



Einstein and Grossman, in their mathematical inquiry 

 into the theory * of gravitation, deduce the existence of 

 such an effect, the order of magnitude of which is so small 

 that it cannot be observed by any known apparatus. 



In 1912 the authors constructed at the Manchester School 

 of Technology the apparatus shown in fig. 2. There were 

 special facilities existing at the school for carrying out this 

 work. A heavy steel flywheel and an electric motor for 

 driving it had already been constructed for other work ; 

 moreover, the School building afforded a very rigid support 

 for the suspension, as it forms a hollow rectangle 90 metres 

 by 60 metres, not very easily deflected by a torsional stress. 

 The well of a lift provided a very suitable place to hang a 

 long delicate suspension. 



* See Fokker, Phil. Mag. [6] xxix. pp. 77-96. 





Phil. Mag. S. 6. Vol. 32. No. 192. Dec. 19KJ 



2 S 



