6 



IM. ( '. Chree. ///'< *ljations on 



attainable. Dr. Hanker tried the effect of greater tightness This, 

 however, made matters rapidly worse, and it shortly became obvious 

 that the box in its existing condition was of no fuither use; it was 

 accordingly sent for repair to Cambridge. 



Having high hopes of a new system of plug holders fusible metal 

 inside Doulton ware the Instrument Company introduced this, at their 

 own expense. The plug holders in the restored box are supported 

 tely, and the pulling out of a plug has never shown any ten- 

 dency to influence others. This is undoubtedly a great improvement ; 

 but I am somewhat doubtful of the expecb'ency of the other changes 

 made when restoring the box. 



After some time it was found that taking out and replacing a plug 

 sometimes exerted a very sensible influence on the reading, and from 

 the appearance of the plugs it was suspected that this arose partly 

 from the state of the plug holes. The fusible metal seems disposed to 

 develop a coating of light-coloured powder, and presumably this 

 affects the plug resistances. On being applied to, the Instrument 

 Company supplied a simple arrangement for cleaning out the plug 

 holes without undue friction, and it has certainly improved matters. 



The accompanying sketch, fig. 1, shows diagrammatically a vertical 

 section of the original box, perpendicular to its longest dimension. 

 C represents the coil chamber, AAA a copper tank containing water. 

 This tank could be heated from below by a gas burner, the flow of gas 

 being determined by a gas regulator, whose mercury bulb was sur- 

 rounded by the water inside A. The coils hung in air, and their 

 temperature was deduced by means of a mercury thermometer whose 

 bulb was inside C. 



When altering the plug system, the Instrument Company altered 

 the shape of the water tank and the form of the coil chamber. Fig. 2 

 gives a section of the existing form taken in the same way as fig. 1, 



FIG. 2. 



the letters having the same significance. The changes were presumably 

 dictated by the altered nature of the plug arrangements. I am dis- 

 posed to think that the coil chamber in the existing form is not so well 

 protected from external influences as in the earlier form, and I believe 

 that in accepting the readings of the mercury thermometer, as giving 

 the temperature of the coils, there is more risk of error now than 

 formerly. 



