Bare Wire for Resistance-coils. 215 



order to diminish as far as possible the effects of the vibration 

 caused by London traffic, thick rubber blocks are placed on 

 the pier supporting the galvanometer ; this precaution has 

 made it possible to work at any time except when large 

 masses of iron are being moved in the neighbouring goods 

 station. The scale is placed 3 metres from the mirror. The 

 motion of the mirror is observed by means of a small electric 

 lamp which projects the image of a fine wire on to the mirror, 

 thence it is reflected on to a ground-glass scale. The observer 

 is at a considerable distance from the galvanometer, a matter 

 of some convenience since the zero is thus not so liable to be 

 changed. The galvanometer gives a distinct movement for 

 an alteration of '00001 of an ohm, with 10 ohms in every 

 arm, the battery being one Hellesen cell, together with an 

 added resistance in series with it of 10 ohms. The galva- 

 nometer is connected to the bridge by 20 gauge rubber- 

 covered high insulation wires ; the rubber covering is neces- 

 sary as I have found serious errors introduced unless the 

 insulation of the galvanometer connexion is extremely good. 



IV. Determination of Standards. 



The temperature difficulties which are most liable to cause 

 inaccuracy in resistance measurements have been greatly 

 simplified by the remarkable uniformity of temperature of 

 the room in which the measurements have been made. The 

 room is almost entirely underground, and is protected on all 

 sides by other portions of the building from the heat of the 

 sun. It is lighted by one small electric lamp, which gives 

 rise to no appreciable change of temperature. The presence 

 of one experimenter causes the temperature of the air to rise 

 about *2 of a degree during the first half hour, and then it 

 becomes constant ; but the temperature of the resistance-box 

 and standards, the latter being placed in earthenware vessels 

 and surrounded by sawdust, takes several hours to rise as 

 much as *03 of a degree. 



The bridge-wire was calibrated by a method due, I believe, 

 to Mr. Griffiths. In one of the outer arms is placed, by means 

 of mercury-cups, a coil of low resistance, called the gauge- 

 coil ; on the other hand, the gap can be short-circuited by 

 means of a thick copper bar. In the opposite outer arms are 

 placed three resistance-boxes in parallel. The object of this 

 arrangement is to secure the necessary fine adjustment. 

 Commencing with the slider at one end of the wire, the 

 gauge-coil being in place, a balance is obtained by adjusting 

 the resistance in the boxes, the slider remaining fixed. The 



