CONDUCTIVITY BOXES. 



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experiment shows that they may alter in the course of time, but 

 they are very little affected by changes of temperature.* 



928. CONDUCTIVITY BOXES. Sir W. Thomson has given this 

 name to a system of coils arranged so as to compare directly the 

 inverses of resistance that is to say, conductivities.! 



When several resistances r lt 



r nJ are arranged in multiple 



arc between two points (209) the conductivity of the system, or the 

 inverse of its resistance R, is equal to the sum of the conductivities 

 of each of the arcs. 



iii i 



-=-+- + ....+. 



Consider, for instance, a series of coils the resistances of which vary 

 as powers of 2. All the coils are connected at one end with the 



Fig 182. 



same bar of brass, A A' Fig. 182, while the other end terminates 

 in a copper block. These blocks are at a small distance from a 

 second similar bar, B B', parallel to the first, and with, which they 

 may be connected by plugs. In this way we introduce between 

 the two bars, and therefore between the points A and B of the 

 circuit, as many coils in parallel arc as there are shunts. 



The figures of the top bar represent the resistances of each coil, 

 and the lower numbers represent the conductivities multiplied by 16. 

 With the plugs as arranged in the figure, the conductivity of the 



22 l6 



system would be -, and its resistance 



16 



22 



* WERNER SIEMENS. Reproduction of the Unit of Resistance. 1882. 



t Sir W. Thomson has proposed to designate by mho, which is the word 

 ohm reversed, the conductivity of a body whose resistance is an ohm. A mhometre 

 is an apparatus for measuring conductivities. 



VOL. II. Y 



