554 



ELECTRICAL MEASUREMENTS 



the paper. The trough holds a few drops of an aniline-glycerine 

 ink which is carried to the paper by capillary action. As the 

 reservoir holds but little ink, frequent attention by the operator 

 is necessary. 



Another instrument of the same class is shown in Fig. 339A. 

 A counterbalanced soft iron core, consisting of a tube which pro- 

 jects perpendicularly from a disc of the same material, is at- 

 tracted into the coil against the action of a spiral spring. By a 



FIG. 338. Bristol curve-drawing ammeter. 



simple lever the motion of the core causes the pen to travel over 

 the chart. The tube and disc are slit to reduce eddy currents. 

 An adjustable iron plug in the lower part of the solenoid allows 

 the deflection at the upper end of the scale to be adjusted. The 

 needle is damped by an aluminum damping disc of the usual 

 form actuated by gearing from the pivot carrying the index. 

 The clock is ordinarily arranged so that the chart is for either a 

 12- or a 24-hour period. 



