138 



Prof. H. L. Callendar and Mr. H. T. Barnes. 



exceeded by one-tenth of a millivolt that of all the other crystal- cells 

 we have tested. Even in this case the cell was at least consistent 

 with itself. It was first tested at 15°, 30°, 15°, 0°, and 15° 0., allow- 

 ing a day at each temperature. It was a new cell, the first of this 

 type which we tested, and its value at the time was nearly three- 

 tenths of a millivolt higher than the standard. The value found at 

 0° C. was 16 72 millivolts above, and at 30° C. 1972 millivolts below 

 its value at 15° C, instead of +16'62, and — 19"58 millivolts respec- 

 tively. The cell being of a very small and sensitive form, the test 

 was repeated a feAv days later in the reverse order, allowing only 

 half an hour at each temperature. The values found in the second 

 test were +16'71 and — 19'70 millivolts respectively. We thought 

 at first that the discrepancy might be due to some inherent peculi- 

 arity of this type of cell. We have since tested other cells of the 

 same type and size, with results which agree to 0*02 millivolt with 

 the B.O.T. crystal cells. The agreement is not confined to points 0° 

 and 30° C. For instance, the difference found at 24° C. was O05 of 

 a millivolt less thau that calculated by the formula (P). The B.O.T. 

 crystal cells at this point show a difference of 0*04 millivolt from the 

 curve (P) in the same direction. We consider that the divergence 

 of one-tenth of a millivolt in the case of this particular cell must be 

 regarded as exceptional. 



§ 19. Importance of Constant Conditions of Temperature. 



We have quoted the above test partly as an illustration of the kind 

 of agreement between cells of different types which it is possible to 

 attain with suitable cells under definitely known conditions of tem- 

 perature. 



In attaining this order of accuracy the chief difficulty lies in the 

 certain determination of the temperature of the cells. To attain a 

 certainty of the order of 0*01° C, the following conditions are 

 necessary : — 



1. The cells should be of an elongated form, and should be deeply 

 immersed in a bath of liquid, which is constantly and vigorously 

 stirred. 



2. The temperature must be kept constant to 0"01° C, and the 

 thermometer used must be read and corrected to the same order. 



If the cells are not deeply immersed their temperature will be 

 affected by external conditions. If the liquid is not constantly 

 stirred it will tend to be hotter at the upper surface, especially if the 

 liquid be very expansible, like paraffin, and the bath be hotter or 

 colder than its surroundings. If the temperature is changing the 

 <3ells will not be of an uniform temperature throughout, and will lag 

 behind the thermometer, unless they happen to be of a smaller size. 



