270 Mr. C. W. Siemens on a Resistance-measurer. 



it to the sea. It is probably being fast removed also from the 

 New Red Sandstone; for although the analyses of February 

 show that the Severn does dissolve this substance from the 

 New Red Sandstone itself, yet they prove that it is chiefly from 

 the Salwarp, which directly drains the Red Marl (in which the 

 great deposits of rock-salt lie), that it acquires the large quantity 

 of chloride of sodium which it contains at Worcester. With 

 regard to its earthy carbonates it also presents some peculiarities. 

 As contrasted with the Thames, it contains only about half the 

 quantity of carbonate of lime, and about double the amount of 

 carbonate of magnesia : the want of the former constituent is 

 doubtless due to the comparative absence of limestone rocks from 

 the upper channels of the Severn, while the magnesic salt pro- 

 bably arises from the decomposition of silicates, which may also 

 be the source of the potassium which the water contains in rather 

 large quantity. 



XXXIV. On a Resistance-measurer. 

 %C.W. Siemens, F.R.S.* 



FOR the measurement of small resistances the method for- 

 merly employed was that of the tangent galvanometer, 

 which method is still valuable in the determination of resistances 

 which are inseparable from a difference of electric potential, such, 

 for instance, as a galvanic element. 



In measuring wire-resistance, more accurate and convenient 

 methods have been devised, amongst which that of the common 

 differential galvanometer and that known as Wheatstone's ba- 

 lance hold the most prominent places. 



But both these systems have disadvantages which render them 

 insufficient in a great many cases. For instance, in the first me- 

 thod a well adjusted variable-resistance-coil is necessary, which, 

 if the method is intended to be applicable between wide limits, 

 will have impracticably large dimensions. The bridge method, 

 though very beautiful, requires three adjusted coils, and fre- 

 quently gives rise to calculation, which renders it unavailable for 

 unskilled operators. The sine method, which is the most suit- 

 able for measuring great resistances, requires even a superior 

 amount of skill and mathematical knowledge on the part of the 

 operator. Many years' experience of these methods made me 

 feel the want of an instrument which would, by its simplicity of 

 construction and ease of manipulation, be capable of employment 

 by an unskilled operator with a degree of correctness equal to 

 that of the bridge method. 



* Communicated through the Electric- Standard Committee. 



