Platinum Thermometry'at Kcw Observatory. 17 



same side as it goes to when the plugs are dirty and have an increased 

 resistance. The amount of the drift is very variable. One day it may 

 be negligible, while the next day it may be as much as 0'04 ; it is 

 occasionally more than this. The phenomenon is probably partly due 

 to the fact that frequent pulling out and putting in of the plugs seems 

 inclined to produce the grey powder already referred to. It may also 

 arise from the observer's standard of plug tightness falling off as he 

 becomes tired, or from asymmetric heating of the box through the 

 proximity of the observer's person to the minus side of the scale. There 

 are probably various influences at work, some of which may be peculiar 

 to the particular box. 



One cannot be perpetually taking bridge centres, so that even the 

 more recent observations are exposed to some uncertainty from this 

 source. 



The practice of regularly taking bridge centres was not introduced 

 until 1897, and it is of course possible that there was little or no 

 occasion for it in the original box with brass plug holes. 



Thermo-electric Currents. 



16. The principal seat of thermo-electric currents seems to be the junc- 

 tions at the head of the thermometer ; and it is desirable to shield the 

 head as much as possible from heated air or vapour. When a Griffiths 

 key is used these currents are not necessarily a source of error, but they 

 . tend to increase the difficulty of taking readings. Originally it was sup- 

 posed that the use of a Griffiths key rendered further care unnecessary, 

 and no commutator was used until after the restoration of the box in 

 1897. As the Instrument Company then transferred certain terminals to 

 the key-board, it is possible that the original box suffered but little if at 

 all from the source of trouble now to be described. 



When the commutator was introduced the following phenomena were 

 observed. When the day's observations began the readings with the 

 current d and r (direct and reversed) were usually about the same. 

 After a few minutes' observations the difference between the d and r 

 readings began to increase, and nearly always in one direction. After 

 a little the difference usually assumed a fairly constant value, but con- 

 siderable fluctuations might occur, especially if the temperature of the 

 room was rendered unsteady. On the advice of Mr. W. N. Shaw the 

 thermo-electric key was enclosed in a padded box. This has decidedly 

 diminished the evil, but it still appears expedient to take readings with 

 the current both ways. Table VI shows the state of matters typical 

 before and after the introduction of the protecting box. 



When the thermo-electric effect is not eliminated the error in the 

 reading is (d cx< r)/2. 



Supposing d-r to remain constant through the ice, steam, and 



VOL. LXVII. C 



