1 1; Dr. < '. < 'hive. / "u 



\ nmplete calibration is a laboi ions process, and its frequent repeti- 

 tion would add seriously to the work entailed by platinum tlu-:iiio- 

 metry. If, however, the results in Table V are normal, it would appear 

 that frequent calibrations can hardly be avoided in the case of physical 



k of the highest accuracy. 



Faulty Action of the Contact Piece. 



14. Parallel to the real bridge wire in the Kew resistam box runs 

 an exactly similar wire connected to the galvanometer, and the contact 

 piece works by pushing down a short cross wire so as to span the 

 interval between the wires. In January, 1898, it was noticed that 

 mere moving and resetting the contact piece might alter the reading, 

 and I found that the pressure to which the cross wire was exposed 

 when contact was made always acting on the same part of its surface 

 had cut a groove in it. The Instrument Company put this to rights, 

 but the phenomenon had repeated itself by January, 1899, though to a 

 smaller extent. On that occasion the Instalment Company made an 

 alteration which, it is hoped, will prevent the recurrence of this trouble. 

 It is difficult to keep the two parallel wires equally tight and exactly at 

 the same horizontal level ; thus the cross wire may bear unduly heavily 

 on one wire before it makes good contact with the other. This defect 

 would be of little consequence in an open scale bridge wire ; but in the 

 Kew box it produced, when at its worst, uncertainties of the order 

 -04, or even 0'05, in individual readings. Of course this merely 

 tended to introduce irregularity in the readings, and supposing a 

 number of observations taken, could hardly simulate a change in a 

 thermometer. 



Shift of the Bridge Centre. 



S 15. By the bridge centre I mean the vernier reading when a balance 

 is made with all the plugs in their holes, the platinum thermometer and 

 compensator resistances being cut out by short-circuiting straps. 



The original departure 0*006 in this centre from zero on the scale 

 was given as a fixed correction in the first calibration table, and as no 

 provision existed for determining the centre, I did not for some time 

 properly appreciate the situation. My attention was first roused by a 

 sudden apparent discontinuity in the values of RQ in the spring of 1897, 

 for which the only apparent cause was a cleaning up of the box. The 

 Instrument Company then supplied two short-circuiting straps, to be put 

 across the CC and PP box terminals, and since then determinations of 

 the centre have been made at the beginning of each observation day, 

 and usually at intervals throughout it. 



Soon it became apparent that during a day's observations the 

 bridge centre is apt to drift towards the mimis side of the scale ; the 



