276 ON THE RIGIDITY 



In most of the measurements made, the quantity under con- 

 sideration was determined as a mean of several observations. 



Lengths were measured by means of a beam compass reading 

 to .01 inch. The limit of error of a mean value was found by 

 comparing a number of such mean values with their mean, the 

 greatest divergence being taken to be the possible error of a 

 determination. It was found to be different according as it was 

 the length of the cord or of a side of the Mai lock triangle or 

 of the arm, that was determined. In the two former cases the 

 greatest divergence from the mean was .01 in., in the latter 

 .005 in. 



The diameters of the cords, which were approximately 

 cylindrical, were found by means of a screw-gauge reading 

 to .001 inch. The possible error was found by the above 

 method to be .0005 in. Owing to the difference in diameter at 

 different parts of the cord, it was found necessary to have 

 marks on the cord, at which the measurements were always 

 made. 



The error that might be made in determining the angle of 

 twist in the static method was estimated to be about a quarter 

 of a degree. 



The weight of the small plummet used in twisting the cord 

 was found by means of a balance weighing to .001 grm. The 

 method of weighing was that of substitution, and the limit of 

 error was estimated to be .0005 grm. 



The time of oscillation was found by means of a stop-watch 

 divided into fifth-seconds, but capable of estimation to .1 sec. 

 The limit of error was determined in the same way as in the 

 case of length, and found to be about .04 sec. 



In the static method the formula used for the calculation of 

 the rigidity was the following : n = 2 Tl/^r *o in which T is the 

 torque in Ib.-inch units, applied at lower end of cord, I is length 

 of cord in inches, r is radius of cord in inches, and f> is angle 

 twisted through measured in radians. For the kinetic method 

 the formula : n = S^lI/t 2 r*g was used, in which I and r were 



