258 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The characteristic features of the relations between twist and strength 
shown in the above tables and diagrams are : — In single threads the 
strength of the thread without twist is very small ; there is not much of 
an increase in strength during the first turn ; immediately afterwards the 
thread commences to get stronger at a rapidly increasing rate, but after a 
few turns the rate of change of strength begins to decrease, the curve 
approaching an asymptote. This asymptotic strength is much less than 
the ultimate breaking strength of 1 millimetre of thread. 
The relations for two-ply threads have similar characteristics to those 
for single threads, but the initial strength in each case is considerable. 
During the first one or two turns of twist, two processes are in operation 
affecting the strength of the thread. The two-ply twist strengthens the 
thread, but simultaneously with this action the twist in the single threads is 
being opened out to some extent ; the latter process weakens the thread. 
After the first turn per inch, the opening-out action becomes of little 
account. It is probable that the thread may be actually weakened in the 
initial stages of the second twisting. The above experiments were not 
sufficiently extensive and refined in measurement in the initial stages to 
settle this point. The question is of no practical importance, as the twist 
impressed on two-ply threads is always greatly in excess of the limit of 
twist within which the strength of the thread is affected by the opening 
out of the singles. For a similar reason the direct twist side of the diagram 
was not investigated; two-ply yarn is never twisted for commercial purposes, 
in the same direction as the twist in the singles. The investigation there- 
fore deals with the inverse twist in two-ply threads. Previous to the 
spinning operation, single yarn comes from the condenser in a thread-like 
sliver, but possessing no twist. The relation between strength and twist 
will not be changed by the direction of the twist, whether “ crossband ” or 
“ openband.” 
When the twist becomes excessive the determination of the yield point 
is beset with great difficulties. Frequently the thread breaks with a slightly 
lower load than it carried with a lesser degree of twist. It does not follow,, 
however, that this is due to a weakening effect of increased twist at this, 
point. The reason is rather that the thread becomes weaker by the con- 
tinued application of tensile stress. Again, very “ hard ” threads break 
suddenly without giving much evidence of a yield point. As such ex- 
cessively twisted threads are too “ hard ” for use in any textile fabric, the 
exact character of the relation holding between strength and twist in these 
did not appear to be of sufficient importance to demand very careful in- 
vestigation. But the striking fact remains that, although twist is the most, 
