434 Mr. T. Mather on the Shape of Movable Coils 



facts at his disposal*, yet the hydrates which I mention as 

 existing include those which both Mendeleeff and Crompton 

 mentioned, thus: — 



From Densities. 



From Conductivities. 



Mean from all 

 sources. 



Mendeleeff. 



Pickering. 



Crompton. 



Pickering. 

 At 84-0 



Pickering. 



About 84*5 



At 84-48 



About 84-5 



84-24 per cent. 



n 731 



„ 73-07 



„ 731 



„ 73(?) 



73-04 



„ 476 



„ 50-33 



„ 47-6 





49 92 





„ 18-5 



„ 18-5 





18-92 



„ 3-5 



„ 4-15 



„ 3-5 



„ 3-5 



3-99 



I have mentioned above that the second differential is recti- 

 linear in every case ; but I do not by any means think that it 

 has been proved to be absolutely so. The investigation was 

 purely experimental in its nature ; and the point of chief 

 importance was that the lines ultimately obtained by differen- 

 tiation should be straight within the limits of experimental 

 error, for their being so proves that each of them is derived 

 from an independent curve, which, within these same limits, is 

 regular. 



XL VI. On the Shape of Movable Coils used in Electrical 

 Measuring-Instruments. By T. Mather, Assistant in the 

 Physical Department, Central Institution] '. 



IT is with some diffidence that I venture to bring the sub- 

 ject of this note before the Society, because it concerns 

 such a fundamental point in the construction of measuring- 

 instruments having movable coils, such as d'Arsonval galva- 

 nometers, electro-dynamometers, wattmeters, &c, that it is 

 almost certain to have been worked out before. However, as 

 the matter is not touched upon in the ordinary text-books, I 

 bring it forward in the hope that it may serve to recall atten- 

 tion to a subject which, judging from the construction of such 



* It is not difficult to see how both Mendeleeff and Crompton arrived 

 at partially right conclusions from erroneous or insufficient premisses ( loc 

 cit. pp. 79, 86, 125). 



t Communicated by the Physical Society : read March 21, 1890. 



