﻿118 Prof. 0. Henrici on a 



point P, then the spindle will have turned through an angle 

 n6, where 6 corresponds to the x of P. 



If, now, the spindle had at its lower end two registering- 

 wheels at right angles to each other rolling on the drawing- 

 paper, we should have in principle my old model (§4) with 

 {Sharp's inversion. Instead of this Coradi gave each spindle one 

 registering-wheel and made this roll on the cylinder C. This 

 requires for each registering- wheel a separate spindle, hence 

 two for each pair of coefficients A» and B n . It substitutes, 

 however, the rolling on a smooth surface for that on the rough 

 surface of the paper. The instrument made according to 

 this design for the Guilds' Central Technical College has five 

 such pairs, so that on going once over the curve the first five 

 pairs A n and B n are obtained. The extra spindle which is 

 driven by the silver wire contains, however, three extra disks, 

 making lour in all. If the wire is stretched over the top disk 

 we get, as stated, the coefficients for n = l, 2, 3, 4, 5. The 

 second pulley has half the diameter, iso that the spindles turn 

 twice as fast if the wire is stretched round it. Thus in going 

 over the curve a second time we get the new coefficients for 

 n = 6, 8, 10. The remaining two disks give similarly the 

 coefficients for n = l and 9 respectively. Hence on going 

 four times over the curve we get ten pairs of coefficients. In 

 most cases the five pairs obtained at once will be amply 

 sufficient. 



For the details of the construction I must again refer to 

 Prof. Dyck's Catalogue (Nachtrag, p. 34) and only mention 

 a few points. The axis of a registering-wheel lies in the 

 diameter of a horizontal ring which is attached to the lower 

 end of the spindle by aid of an elastic vertical steel plate. 

 This presses the wheel against the cylinder, securing contact. 

 On testing the instrument it was found that this plate was 

 liable to slight torsion which affected the readings. It 

 showed a number of other drawbacks of more or less im- 

 portance. One is that the registering-wheel not only rolls 

 but also slips. This slipping is absent in the Analyser of 

 Lord Kelvin, who has dwelt strongly on the importance of 

 avoiding it. 



There was also a serious difficulty in taking the readings. 

 The instrument registers up to 20 centim. If the zero-point 

 has passed the index which gives the reading, 20 centim. 

 have to be added or subtracted. Every one who has used a 

 planimeter is accustomed to this, and knows how to take 

 account of it, for he can either estimate the area sufficiently 

 to see which correction is necessary, or he can go rapidly 

 over the curve again, watching the zero-point. Neither 

 method is possible with an Analyser which gives a large 



