ALTERNATING CURRENTS 



TANGENT SCRE 



COACH-SPRINO 



voltmeter, the construction of the working parts of which will be 

 understood by a study of Figs. 43 and 44. In Fig. 43 is shown the 

 movable system of vanes, or " needles," which are threaded on a rod 

 carrying a damping vane at its lower end, and suspended by a fine wire 

 from a coach-spring attached to a torsion-head provided with a tangent 



screw for adjusting the zero of 

 the instrument. The object of 

 using a coach-spring between the 

 torsion-head and the suspension 

 is to prevent the latter from being 

 broken by a sudden accidental 

 jerk or blow. If, in fact, the vanes 

 are jerked downwards, the coach- 

 spring yields sufficiently to allow 

 the safety-sleeve to come into 

 contact with the guide-stop before 

 the suspending wire is over- 

 strained. By means of a clamp- 

 ing-spring, which may be raised 

 by a screw provided with a milled 

 head which projects through the 

 bottom part of the case, the 

 movable system may be raised 

 until the conical portion shown 

 above the pointer comes against 

 the guide-stop. The suspension 

 is then quite slack, and the in- 

 strument is capable of standing 

 a considerable amount of ill 

 usage without damage. The per- 

 forated damping-vane fits into a 

 glass vessel filled with oil, the 

 oil being at a sufficiently high 

 level to completely cover the 

 vane. 



The movable vanes are in 

 connection with one terminal of 

 the instrument. The other ter- 

 minal is connected to the two 

 one set on each side of the movable plates, 

 The fixed plates are not shown in Fig. 43 

 Each set of fixed plates contains one 



*?K.OVA3LE PLATES 



fOR CLAMPIN& 

 MOVABLE StSTEM 



DAMPING MANE 



FIG. 43. Multicellular Voltmeter. 



plates, 



Fig. 44. 



sets of fixed 



as shown in 



for the sake of simplicity. 



more plate than the movable system, there being a fixed plate above 



each half of the uppermost moving plate, and a fixed plate below 



each half of the lowermost moving plate. 



