126 THE SCIENTIFIC PAPERS OF 



between the wires, the results registered would at once show such 

 utter discordance with probable facts, that attention would be 

 drawn to it. Of course, the instrument could be destroyed if 

 -exposed to intense heat ; but in that case it would tell its own tale. 



Mr. Bell : I should like to ask Mr. Siemens what he estimates 

 would be the cost of this instrument ? That is rather an impor- 

 tant thing. 



Mr. Siemens : I can hardly answer that question in a very 

 decided manner, your lordship ; because this instrument, which is, 

 as it were, the mother instrument, has been very expensive. 

 Another complete instrument would cost, I think, about 16 

 including one or two pyrometer coils, which latter are not expensive 

 except for very high temperature, when cases of platinum have to 

 be resorted to. A platinum tube would probably cost about 10 ; 

 but if an iron or copper tube, such as would suffice for a 

 temperature below whiteness, the cost in this part of the instru- 

 ment is but trifling. 



In the discussion of the Paper 



"ON A MODIFIED FORM OF < WHEATSTONFAS 

 BRIDGE,' AND METHODS OF MEASURING SMALL 

 RESISTANCES," by PROF. G. 0. FOSTER, 



THE PRESIDENT * (Mr. C. W. Siemens) said, having been con- 

 nected more or less with the Wheatstone bridge in its early appli- 

 cation to telegraphic measurement, he would say that the instru- 

 ment was first attempted to be used in testing coils of insulated 

 line wire by his brother in 1847-48. It was soon found that the 

 range of the instrument was insufficient, and it occurred to his 

 brother to construct these resistance-boxes so as to make the two 

 arms of the balance variable, which gave a much larger range of 

 reading. Instead of simply adjusting one weight with another on 

 equal arms of the balance, he made, so to speak, the length of the 

 arms variable, and got thereby much wider limits within which 



* Excerpt Journal of the Society of Telegraph Engineers, Vol. 1. 1872-73. p. 207. 



