506 Prof. A. Anderson on Neumann s 



current damping, which was further shown by contracting 

 the tin-plate cylinder still further, and placing it inside the 

 frame, where it again had the effect of largely reducing the 

 strength of the signals. That damping enters largely into 

 the matter was also proved by placing a large flat copper 

 plate close up to one side of the frame, where it could have 

 no screening, but only a damping effect, which also greatly 

 diminished the strength of the signal?. 



The result obtained with the frame inside the copper box, 

 in getting signals of equal strength with the open side of the 

 box facing away from the source of the waves, as when the 

 open side faced towards such source, may perhaps be of some 

 interest, from the point of view of theory, in throwing some 

 light upon the mechanism of electromagnetic wave pheno- 

 mena, inasmuch as it gives evidence that, in addition to 

 what is analogous to a "push " upon an aerial in front of the 

 wave front, there is also something of the nature of a " pull 3y 

 upon an aerial behind the wave front. 



Otherwise the experiments go to show that, at any rate 

 upon the small scale employed, the use of a shielding tube 

 or box does not assist in improving directional reception. 

 It is possible, however, that if tried on a larger scale, with 

 much more space between the frame and the shield so as to 

 avoid damping, the results might be different. 



LIX. Note on Neumann s Law of Electromagnetic 

 Induction. B;j Prof. A. ANDERSON *. 



"HEX a closed conducting circuit is moved in a mag- 

 netic field, a current flows round it, the value of 

 the electromotive force producing the current being given 

 by Neumann's Law. This law is established quantitatively 

 from dynamical principles, but I think one must feel that 

 the proof is not altogether free from objection. For, as it 

 is usually presented, an electromotive force E is supposed to 

 be already in the circuit and then, by the conservation of 

 energy, 



EC-C 2 P + C^. 

 at 



The circuit, however, may be moved so that C is always 

 zero, in which case nothing can be deduced from this 

 equation. If = at anv instant and if it varies in value 



dTS 



during the motion we must have E = — r- at that instant. If 



at 



* Communicated bv the Author. 



