On the Use of Bare Wire for Resistance-coils. 213 



plugs are connected with the zero socket of the previous dial 

 by means of heavy flexible leads, having a copper core, 5 

 milliin. in diameter, composed of copper wires of diameter 

 •2 of a milliin. It was found necessary to use comparatively 

 large wire, as the first set of connecting leads, built up of 

 wire *07 millim. in diameter, parted with the constant 

 turning and gave rise to a variable plug-resistance. The 

 use of the conical plug in a solid conical socket is completely 

 free from errors caused by the change of shape of the insu- 

 lating material ; further, such forms can be easily and accu- 

 rately machined, a matter of no small importance to those 

 who make their own apparatus. 



The terminals are of large diameter (25 millim.), and are 

 arranged with the screw in the moveable part, so that the 

 top of the fixed part can be readily cleaned. 



The proportional arms are fitted with the usual divided 

 blocks. 



The whole of the metallic part is made of phosphor-bronze, 

 a material which is harder and less liable to oxidation than 

 ordinary brass. 



The containing box is lined with copper for the reception 

 of the oil. 



The form of bobbin and the wire for the standard resistance- 

 coils are the same in the 1 and 10 ohm standards ; but in 

 the 100 and 1000 ohm standards the mica is stiffened by 

 being slipped into a brass back, like the back of an ordinary 

 tenon saw, which quite prevents any tendency to bend. The 

 temperature is measured by means of a thermometer placed 

 in a pierced tube which lies inside the coil. The coils are 

 inclosed in an outer brass case and fitted with the usual 

 copper terminals for use with mercury-cup connexions. 



The t J q ohm standard is of manganin, of the usual British- 

 Association pattern. 



The oil employed is a pure heavy hydrocarbon oil, obtained 

 from Messrs. Price's Patent Candle Company ; it is free 

 from any trace of alkali or acid, and experiments conducted 

 by Messrs. Price prove that it has no action on the wire. 



III. Construction of Bridge and Galvanometer. 



The bridge employed was of the usual equal-armed pattern, 

 having a straight wire of platino-iridinm 1*5 millim. in dia- 

 meter and 500 millim. long. The wire is stretched imme- 

 diately over the scale by means of two brass split chucks, 

 these chucks terminating in circular brass rods, having a 

 diameter of 6 millim. The rods slide in heavv brass blocks. 



