AUXILIARY APPARATUS 



419 



seen, indicating pointer is apparent when it is remembered 

 that one operator may have to notice continually the indi- 

 cations of a hundred or more of these meters. 



Damping of a Meter. It is very essential that a meter 

 be well damped; otherwise the finger will be continually 

 oscillating back and forth and the operator must guess at 

 the proper reading. Air vanes on the moving element, 

 or the reactions of eddy currents in the moving element, 

 serve to prevent the oscillation of the finger. If the 

 meter is well damped it may be connected to a circuit and 

 the needle will almost immediately come to rest in its proper 

 position. An example of a well-damped instrument is 

 shown in Fig. 274. On being connected to a circuit the 

 finger overshot its proper position (3) by a very small 

 amount (to position 2) and immediately dropped back to 

 its proper position. It is not advisable to have the damp- 

 ing desiii-nrd so well that the meter overshoots not at all 

 because then the operator would have difficulty in ascertain- 

 ing whether or not the meter 

 were sticking and so not 

 indicating accurately. 



Portable Meters. The 

 portable type of meter differs 

 from the switchboard type in 

 that it is licncrally more accu- 

 rately calibrated, has a more 

 accurate and finely divided 

 scale, and has a very thin in- 

 dirating pointer. A switch- 

 board instrument which indi- 

 cates with an accuracy of 1% 



is generally plenty good enough; for laboratory tests, how- 

 ever, much higher accuracy is generally required. A com- 

 mon type of portable laboratory voltmeter is shown^in 

 Fig. 275; it is mounted in a box and fitted with a carrying 

 handle. 



FIG. 275. A Portable Type Volt- 

 meter, for Laboratory Use. 

 Weston Electrical Instrument Co. 



