in Incandescence Lamps. 143 



side is covered up by the shower from the clamp on that side. 

 It has been noticed in one or two cases that small tufts of 

 carbon are seen on one clamp, and that when a well-defined 

 shadow exists on that side, this seems to indicate that the 

 shower -of copper molecules has been partly stopped by the 

 opposite clamp, which has therefore acted like a target and 

 become encrusted with a proportion of the molecules shot 

 at it. 



With respect, next, to carbon deposits. Every one knows 

 the appearance of a lamp after it has been burning for some 

 time or overburnt: it is clouded with smoke-like deposit. In 

 nearly all cases of copper deposit the molecular shadow exists, 

 but it is not nearly so often seen in the case of carbon deposits. 

 After many observations it was found that the molecular 

 shadow of the filament, or line of no deposit, could be formed 

 by suddenly raising the filament to a very high temperature, 

 as for instance by placing a 50-volt lamp for an instant on a 

 100- volt circuit ; whereas when the deposit of carbon takes 

 place slowly, and as it were in the natural way, the lamp 

 exhibits only a general smokiness but no line. Again, it has 

 been found that when a carbon filament is cut sharply through 

 at one point, caused by excessive temperature at one spot, 

 there is very frequently a sharply marked line of shadow 

 of the loop on the side of the envelope farthest removed from 

 the fracture. These facts seem to indicate that in normal use, 

 when the lamp is not being pressed beyond the electromotive 

 force at which it was intended to be used, there is a general 

 evaporation of carbon going on from all parts of the loop, and 

 these molecules, being projected with no abnormal velocity, 

 probably collide with molecules of residual air a large number 

 of times before they reach the walls of the envelope, and thus 

 get their trajectories very much changed in direction. In 

 thi3 case the result would be to cause an irregular deposit 

 of molecules of carbon on the glass envelope. But if 

 we suppose a sudden or very excessive temperature to be 

 given to part or the whole of the filament, this may cause a 

 very violent projection of molecules of carbon from the fila- 

 ment; and these would pass outwards in straight lines, and a 

 far larger proportion would reach the envelope in the direction 

 in which they were first projected. This would then cause a 

 deposit on all parts except those shielded by the loop ; and in 

 the case when such violent projection went on from all parts 

 of the loop, as when a lamp is overheated, it is easily seen that 

 parts of the envelope not lying exactly in the plane of the 

 filament would receive twice as much deposit as those exactly 

 in the plane. In most cases of carbon deposits the lamps 



