556 ELECTRICAL MEASUREMENTS 



many cases. The clock should be of good quality, preferably 

 self-winding, the motion of the paper positive, and the time scale 

 uniform. 



The special difficulties in the design of accurate instruments 

 of this sort come from the pen friction which impedes the 

 motion of the pointer. It is best that the scale be uniform and 

 the records given on rectangular coordinates so that they may 

 readily be integrated by a planimeter. A uniform time coordi- 

 nate may be attained by driving the paper by a metal drum 

 having projecting pins which engage in perforations at the edges 

 of the record paper. 



In the better class of instruments the effect of pen friction is 

 minimized or eliminated : 



1. By giving the movable system a high torque and employing 

 a very strong controlling spring. By using soft-iron instruments 

 of proper design, ammeters and voltmeters may thus be con- 

 structed in which the pen-friction error is reduced to 1 or 2 per 

 cent of the full-scale deflection, without an unduly great con- 

 sumption of power. (50 watts in a voltmeter; 7 watts in an 

 ammeter.) 



2. By providing the pointer with a stile that ordinarily swings 

 clear of the paper but which is periodically pressed against it 

 by an electromagnet and imprints a dot. This arrangement is 

 useful where the phenomena under investigation vary slowly. 

 It has proved of great service in those forms of registering thermo- 

 electric pyrometers which are in reality registering millivolt- 

 meters. A modification is to have an arrangement by which a 

 high-tension spark is periodically caused to pass from the stile 

 through the paper, thus again giving the record in the form of 

 dots. 



3. By using the relay principle; in this case the movable system 

 has only to control the position of the pen, the power necessary 

 to move it being supplied from an external source. Relay instru- 

 ments are somewhat complicated, but the wattmeters and the 

 direct-current ammeters as usually designed have a uniform 

 scale and thus give records on rectangular coordinates. 



A direct-action registering wattmeter for polyphase circuits 

 is shown in Fig. 340. Here the friction is overcome by the high 

 torque of the movable systems. 



