442 Mr. H. Tomlinson on the Effect of Stress and 



a superposition of effects due to induction and conduction, 

 the latter being far more difficult to theoretically estimate 

 than the former. But the reasoning regarding the electro- 

 magnetic induction is not very greatly changed, although not 

 so favourable to long-distance signalling. If the return cur- 

 rents diffused themselves uniformly in all directions from the 

 ends of the line, the same property of w-fold increase of M 

 with w-fold lengthening of the lines and their distance 

 would still be true. But the diffusion is one-sided only, and 

 is even then only partial, especially when exceedingly rapid 

 alternations of current take place. But we have the power of 

 counterbalancing this by the multiplication of the variations 

 of current in the primary that we can get by making and 

 breaking the circuit, with a considerable battery-power if 

 necessary, getting something enormous compared with the 

 feeble variations of current in the microphonic circuit, or that 

 can work a telephone. Electrostatic induction also comes 

 in to assist, as it increases the activity of the battery, and 

 therefore the current in the secondary also. 



But, as regards wires connected to earth, this does not pro- 

 fess to be more than the very roughest reasoning, though in 

 my opinion quite plain enough to show that we may ascribe 

 the signalling across 40 miles of country between lines about 50 

 miles long mainly to induction, as we should be necessitated to 

 do if we carried the experiment further and closed the circuits 

 metallically by roundabout courses, for then the plain argu- 

 ment relating to induction will become valid. Experiments 

 of this kind are of the greatest value from the theoretical 

 point of view, and it is to be hoped that they will be greatly 

 extended. 



LI. Note on the Effect of Stress and Strain on the Electrical 

 Resistance of Carbon. By Herbert Tomlinson, B. A* 



PROFESSOB T. 0. MENDENHALL has published in 

 the September number of Silliman's American Journal, 

 and also in the October number of the Philosophical Maga- 

 zine, an account of some experiments on the effect of pressure 

 on the electrical resistance of carbon. These experiments 

 deal not only with such comparatively hard rods of carbon as 

 are used in the arc lamp, but also with the compressed lamp- 

 black seen in Edison's disks. I will first refer to the experi- 

 ments on the hard carbon. Prof. Mendenhall seems to think 

 that these are in accordance w T ith some experiments made by 



* Communicated by the Author. 



