564 



Prof. H. L. Oallendar on Electrical 



zero at one end of the scale, it is necessary to provide a small 

 auxiliary slide for correcting the adjustment when the slide- 

 wire is changed. The freedom of the galvanometer sus- 

 pension from torsion may conveniently be tested at the same 

 time as the zero of the slide-wire, by doubling the resistance 

 in the battery circuit, so as to halve the current through 

 the slide-wire. If this does not produce any appreciable 

 shift of zero, the suspension must be very nearly free from 

 torsion. It is important that the external resistance added 

 should be only in the bridge circuit, and not in that of the 

 relay magnets, which will not work satisfactorily if the P.D. 

 on the relay circuit is much reduced. This mistake has 

 often been made in using this type of recorder. 



Slide-wire Records. 



12. It is possible to obtain records on a more open scale 

 with the slide-wire recorder than with the thread recorder, 

 owing to the greater width of the record sheet, which is 20 cm. 

 for the slide-wire recorder, as compared with 8 cm. for the 

 thread recorder. The latter is limited by the range of the 

 galvanometer boom, which cannot be indefinitely elongated 

 without making the deflexion too sluggish. The record 

 reproduced in fig. 12 was taken with an axillary thermo- 

 meter differing in a few details of construction only from 

 those employed for records 8 and 9. The scale of the original 

 record was 4 cm. per 1° C, which is 2*5 times the scale 

 adopted with the thread recorder. The scale has been 



Fig. 12. 





37°„ „ 



z^- 























t 





















36° 





















J 



1 



2 



j 



Ici.m. 



1 



Y . . . 



t 



) 



,1 



Axillary Thermometer D, on Slide-wire Recorder, 4 cm./deg-. 



reduced to one third, to facilitate reproduction and com- 

 parison. It will be observed that the record shows the same 

 characteristic dip, culminating between 4 and 5 A.M , as 

 records 8 and 9. When first observed, it seemed as though 

 this dip of temperature might be due to some instrumental 

 error, but it has been observed on several occasions with 



