Coherence and Recoherence. 171 
paper * it was shown, by using the electric micrometer, that 
when coherence is broken the surfaces part with a ‘snap ” 
which sounds in the telephone like 
the sundering of a solid body, and 
AS 
Ve 
Y that when the surfaces are pressed 
JZ together again, the ends of the 
bridge ab (formed when the surfaces 
were severed) are found and can be 
easily pressed back flat. 
These facts show that coherence 
is a solid effect, and dispose of the 
ionization theory and also of Branly’s theory. It was shown 
in the former paper } that the strength and resistance of the 
bridge proves it to have about the tenacity of the solid metal 
(for copper T=2x10° c.¢.s.). This again supports the 
fusion theory; and though it does not confute the adhesion 
principle, it requires that the forces of adhesion should be at 
least as great as if the surfaces were fused and _ solidly 
attached to each other. But a simple calculation of the heat 
developed at the contact shows that there is ample to fuse 
the metal. This disposes of the simple adhesion theory ; and 
leaves only the fusion theory, which seems in entire accord 
with all facts known. 
LZ 
LA 
a 
\Y 
\\ 
Recoherence. 
After the cohered surfaces have been sundered, if they are 
brought together again they often cohere with considerable, 
though much less, force than in coherence. This can be 
repeated sometimes once or twice (without further sparking), 
but the power to recohere soon leaves the surfaces, and they 
return to the normal state of inability to cling together. 
This is shown in three distinct cases below. F is the force 
in dynes required to sunder the surfaces before coherence ; 
F, is the force directly after coherence; F,, F;3..., are 
successive values, showing recoherence. 
TABLE I]. 
Case 1. 3 
Jen eter on eee ee ‘Dd dyne ‘D dyne ‘> dyne 
By nccscueeee BGO. 25 BD XD v5 13X°5 5, 
Bean esgauees DOGO? «sy 4D KS, 3 8xX°D 
Eee aS aoe Pin 5 ON: Stage 
in” eee | Cae vine “ie 
i: eercccsesees| 5) He 5 ” | 7 er 
* Ibid. p. 292. t Ibid. p. 282. 
