662 



HENRY A. ROWLAND 



to these waves, and have drawn some diagrams to illustrate the penetra- 

 tion of the alternating current into metal cylinders. The first diagram 

 represents the current at different depths in a copper cylinder, 45 cm. 

 diameter, or an iron one 144 cm. diameter, traversed by an alternating 

 current with 200 reversals per second. The first and second curves 

 show us the current at two different instants of time, and show us how 

 the phase changes as we pass downward into the cylinder. By reference 

 to the third curve we see that it may be even in the opposite direction in 

 the centre of the cylinder from what it is at the surface. The third 

 curve gives us the amplitude of the current oscillations at different 

 depths irrespective of the phase, and it shows us that the current at the 



DIAGRAM 2. 



centre is only about 10 per cent of that at the surface in this case. The 

 second diagram shows us the distribution in the same cylinders when the 

 number of reversals of the current is increased to 1800 per second. Here 

 we see that the disturbance is almost entirely confined to the surface, for 

 at a depth of only 7 mm. the disturbance almost entirely vanishes. 



There are very many practical applications of these theoretical results 

 for electric currents. The most obvious one is to the case of conductors 

 for the alternating currents used in producing the electric light. We 

 find that when these are larger than about half an inch diameter they 

 should be replaced by a number of conductors less than half an inch 

 diameter, or by strips about a quarter of an inch thick, and of any con- 

 venient width. But this is a matter to be attended to by the electric 

 light companies. 



Prof. Oliver J. Lodge has recently, in the British Association, drawn 



