TBANSACTIONS OF SECTION A. 531 



(22) and (23) become 



E=V^ (25) 



1 + e ^ 



and r = l + 2v'{e(l+e)}+2c . . . .(26) 



For good economy r must be but little greater than unity ; hence e must be 



very small, and therefore approximately 



and r = l + 2^/e \ v^'^ 



For example, suppose the resistance of the electro-magnet to be 400 times the 

 resistance of the working coil — that is e = 400 — and we have, approximately, 



-E: = 20i2',andr = l + i 

 That is to say, the resistance in the external circuit is 20 times the resistance of 

 the working coil, and the useful work in the external circuit is approximately \\ 

 of that lost in heating the wire in the dynamo. 



7. On the Application of Eledriciti/ to the Localisation of a Bullet in a 



Woimd. By W. H. Peeece, F.E.S. 



The object of the paper was to show how an electric current can be made an 

 invisible and inmiaterial probe to determine the locality of a bullet in a wound 

 without touching the human body. It was an application of Hughes' Induction 

 Balance, and it had been tentatively used by Professor Graham Bell upon the 

 President of the United States. One pair of coils was made movable, and was 

 used for exploring the body, so as to bring the bullet in the line of the axis of the 

 coils, the depth of the bullet being determined by a second similar bidlet moved in 

 the axis of the fixed coils until equilibrium is obtained. It was sensitive to a 

 depth of five inches. 



8. On some of Bell and Tainter's recent Besearches and tlieir Consequences. 



By W. Lant Carpenter, B.A., F.C.8. 



9. On the Electric Conductivity and Dichroic Absorption of Tourmaline. 

 By Professor Silvanus P. Thompson, B.A., D.Sc. 



Tourmaline possesses electric conductivity differing in difl^erent du-ections, and, 

 according to the author's experiments, conducts better across the optic and crystallo- 

 graphic axis than along it. Tourmaline also possesses the optical property of 

 dichroism, being more opaque, in general, along the axis than across it, equal thick- 

 nesses of crystal being considered, and absorbing the ordinary ray more than the ex- 

 traordinary ; the different rays being unequally absorbed, giving therefore different 

 tints in the two directions. 



In the August number of the ' Philosophical Magazine ' the author has treated 

 the question of opacity of tourmaline by the aid of Maxwell's electromagnetic 

 theory of light, and has re-written Maxwell's equations for the particular case of 

 crystalline media in which fi, the magnetic inductive capacity, K the dielectric 

 capacity, C the volume coefficient of electric conductivity, and V the velocity of 

 propagation of a displacement differed in different directions. He deduces therefrom 

 that uniaxial crystals possessing maximum conductivity along the optic axis wiU 

 absorb the extraordinary ray more than the ordinary ; while those possessing 

 maximum conductivity across the axis will absorb the ordinary ray more than the 

 extraordinary. The former is the case in positive crystals such as smoky quartz 

 and lommellite (magnesic platinocyanide) : the latter' is the case in tourmaline. 

 Maxwell's equations lead to the following formula for the relative intensity of the 

 ordinary and extraordinary rays 



. -M.,C,V., 



lo 



M M 2 



