Standard of Electrical Resistance. 27 



is done, even if dew should collect on tlie outer surface of the 

 funnels, the inner surface is kept dry by the paraffin oil 

 placed in them, the action being the same as that in the well- 

 known Johnson and PhilUp's fluid insulator. 



The ring-coils when in use are placed in rather shallow 

 zinc troughs, which can be filled with water, and which are 

 closed with a wooden lid. When so placed the whole of the 

 actual coil or resistance part is down beneath the liquid at 

 one level, where the temperature can be accurately ascertained. 

 The insulators and point of emergence of the electrodes are 

 away up above the level of the water, and well protected 

 from any action which might permit of leakage over them. 

 The large metallic mass of the ring assists in bringing the 

 resistance-coil quickly back to the temperature of the sur- 

 rounding water, and the coil therefore " tests quickly." In 

 all other respects these standards of resistance are as compact 

 and portable, and not more expensive to construct than the 

 old form of B. A. standard, whilst obviating the difficulties 

 which present themselves in the use of the old form in very 

 accurate comparisons of resistance. 



Other forms of standard coil which have been tried are 

 indicated in figs. 2, 3, and 4, but have not proved themselves 

 to be as convenient as that above described and illustrated in 

 figs. 5 and 6. 



It is quite possible to have two or more coils of wire 

 inside the same ring, each coil having its separate pair of 

 electrodes. A useful coil of this form can be made up con- 

 taining 1, 10, and 100 ohms, so that comparisons can be 

 quickly made at the same temperature with these three mul- 

 tiples of the same unit of resistance. 



The adjustment of the coils to a certain value presents no 

 great difficulties. The wire is in the first instance cut a little 

 longer than required, and its resistance nearly adjusted ; when 

 the two ends of the coil have been soldered to the lower ends 

 of the copper rods, the resistance is again taken from the ends 

 of the electrodes. This resistance should be a little greater 

 than the final value required. The middle point of the wire 

 or extreme loop is now stripped of its silk and the loop 

 twisted up with the pliers, the resistance being carefully taken 

 at intervals. When just a very little in excess of the value 

 required the twisted coil is touched with solder, and having 

 been bound over with insulating material the coil is completed. 

 In the construction of standards it is obvious that it is not 

 so important that the resistance should have an exact integer 

 value at any temperature as that its value at some tempera- 

 ture and its coefficient of variation of temperature should be 

 exactly known. 



