Conductors conveying Steady or Transient Currents. 231 



before long ; but, in common with Prof. Fitzgerald, I feel 

 provisionally and tentatively doubtful whether any mechanical 

 effect really exists between electric pulses travelling along 

 wires with the velocity of light. In a wire subject to electric 

 stationary waves there are obvious electrostatic pulses at either 

 end and electrokinetic pulses in the middle : but Mr. Boys 

 had allowed for all that, and arranged that the opposing effects 

 of ends and middle should conspire to assist each other in 

 causing rotation. What I felt doubtful about was whether 

 even in infinite wires, wherein all complication by reflexion 

 and stationary waves was avoided, a pair of pulses travelling 

 side by side, like a pair of humps (or a hump and a hollow) 

 on a pair of parallel cords, would exert any force on each 

 other. It is known that two charged bodies flying side by 

 side with the velocity of light will exert no such effect (Mr. 

 Heaviside has shown that this is equivalent to saying that two 

 elemeuts in the same wave-front exert no mechanical force on 

 each other); but whether the same thing is true of two wire- 

 conducted pulses has not, so far as I know, been examined by 

 mathematicians. 



If it should turn out that pulses at full speed have no effect, 

 then two straight oscillators in similar phases should repel 

 each other, by the electrostatic effect of the slackening and 

 stationary pulses which are being reflected at the ends. 



Such an action seems optically rather interesting. Max- 

 well predicted that a reflector or absorber would be repelled 

 by light ; though, as we know, the complication of the more 

 vigorous molecular action of material surroundings prevented 

 Mr. Crookes from detecting this precise effect. We know, 

 however, that it must exist ; and the repulsive effects between 

 alternating magnets and copper disks, detected by Faraday 

 and recently made much of in an interesting manner by Prof. 

 ]51ihu Thomson, are examples of this very thing. We can 

 even say what the stress caused by full sunshine ought to be, 

 viz. about 50 microbarads * ; that is, the weight of half a 

 milligramme per square metre : but it has not yet been expe- 

 rimentally observed. If Mr. Boys finds his effect, at least if 

 he finds it in the form I suggest, as an overbalancing static 

 repulsion, it will represent an action between two sources of 

 light or between two similarly illuminated bodies. 



On the afternoon of the meeting of the Physical Society, 



* Langley's recent estimate, that a square centimetre fully exposed to 

 sunshine receives 2'84 C.G.S. thermal units per minute, is equivalent to 

 an energy of 67 ergs per cubic metre of sunshine, or 67 microbarads. (A 

 "barad" means an erg per cubic centimetre, or a dyne per square centi- 

 metre.) 



