1906—7.] Degree of Twist in Single Threads. 267 
both sides of the zero twist. It would therefore be impossible to determine 
the point of maximum length with sufficient accuracy. The following 
modification of the experiment will, however, meet all requirements. 
In fig. 2 the lengths of thread are plotted as ordinates, and correspond- 
ing degrees of twist as abscissae. The zero position 0 is unknown. Let A 
on the axis of x represent x, the turns of twist in the thread, an unknown 
number. Take off a convenient number of turns from the twist of the 
thread, say a, so that the twist can now be represented by point B in the 
diagram. Read the position of the pointer on scale S (fig. 1), and also the 
twist dial. Then turn wheel M quickly in the same direction as before, the 
observer keeping his eye fixed on scale S. The pointer will dip down, 
remain practically stationary for a little, then rise up again, — when the 
Fig. 2. — New Method of ascertaining Twist in Threads. 
pointer comes back to the position on the scale which indicates that the 
thread has the same length as it had with cp — a turns. Let the degree of 
twist be now represented by point C in fig. 2, and that BC = b turns. 
Then the zero position 0 lies half-way between B and C and .’. x = a-\-^ . 
This method is founded on the principle that x turns of inverse twist 
produce the same contraction in a single thread as x turns of direct twist. 
The reason that the twist must be changed quickly through the zero 
position is that the untwisted thread can stand very little stress, and the 
prolonged action of even the light lever F on the thread would produce an 
elongation due to tension which would falsify the reading on the scale. 
The number of turns to bring the pointer back to the same position on the 
scale would be greater than if there had been no stretch due to tension in 
the thread. 
Another method of determining the twist may be arranged with the 
same form of apparatus provided with a heavier lever F. The twist is 
thrown out until the thread breaks. 
