176 



EEPOET 1882. 



As an example has already been given of the determination of 

 secondary points, it is unnecessary to carry the calculation farther. 



Rudberg's Method. 

 Thermometer C. 



(28) A general outline of Rudberg's method has already been given. 

 A thread approximately equal to half the length of the tube is measured 

 with each end in turn at one of the extremities, i.e. in the thermometer 

 used at 100° and 140°. 



Thus, if T be the true thread-length, and t^, t^ the uncollected lengths, 

 and if when the lower end is at 100° the upper end is at 120° + a, and 

 when the upper end is at 140° the lower end is at 120° + /3, two equations 

 are obtained as follows : — 



T = i, + (120 + o) - (100), 

 T = <2 + ^(14O)-0(12O + /3). 



Now, (j> (100) = (p (140) = 0, as these are the standard points, and, as 

 a and ft are small, Rudberg assumed^ (120 + a) = ^ (120 + [3) = f (120), 

 when 2^(120) = i!o-/i. 



The considerable inaccuracies which may be introduced by these 

 assumptions are obviated by the method of transference. 



Thus, since the successive fractions into which the scale may be 

 divided by Rudberg's method are ^, g-, yV, &c., and as in this example it 

 was determined to divide the scale into twelve parts, the principal points 

 are 



100°, 103°-3, 106°7, 110°, 113°-3, 116°- 7, &c. 



In Table XYII., Columns I. and II. give the positions of the lower 



Table XVII. 



Thread I. 



