48 



ELECTRICAL MEASUREMENTS 



necessary in manipulating the instrument not to injure the deli- 

 cate loop or the thermo-couple. 



TABLE OP APPROXIMATE SENSITIVITIES OP THERMO-GALVANOMETERS. 

 DISTANCE 1,000 mm. 



SCALE 



The same principle is applied in the Duddell thermo-ammeter 

 (see page 60). 



POINTS TO BE CONSIDERED WHEN SELECTING A 

 GALVANOMETER 



A galvanometer must be selected with special reference to 

 the work to be done, for no instrument is equally useful under all 

 sorts of conditions. Among the points to be considered are the 

 following. 



Sensitivity. The sensitivity should be sufficient for the work 

 in hand so that measurements may be made without undue 

 fatigue and loss of time. On the other hand, a much higher 

 sensitivity is not an advantage for it means a more delicate 

 instrument and therefore one more liable to injury and more 

 difficult to manipulate. Also, high sensitivity may mean an un- 

 duly long period of vibration and a consequent loss of time in 

 making measurements. Sensitivity, though important, should 

 not be the only thing considered in estimating the utility of a 

 galvanometer. 



Period. The time of vibration of the movable system should 

 be short so that the instrument will respond quickly to the 

 current. 



Damping. When the instrument is in use, the system should, 

 if possible, be critically damped. This will economize time. 



