THE MEASUREMENT OF CURRENT 45 



The resistance of the silvered quartz fiber, which is used in 

 order to attain a sufficiently light " string," may vary between 

 2,000 and 10,000 ohms; with a silver wire about 0.02 mm. in 

 diameter, the resistance is 4 or 5 ohms. 



The electromagnet has a very massive core and the pole pieces 

 are so shaped that they concentrate the field on the narrow air 

 gap in which the "string" moves. The magnet is worked above 

 saturation, consequently small variations of the exciting current 

 have but little effect on the strength of the field in the air gap 

 which is about 20,000 c.g.s. units. 



The time of swing of the movable member of the Einthoven 

 galvanometer is very short; it depends on the size and material 

 of the " string" and the tension upon it. With a fine silvered 

 quartz fiber having a diameter of from 0.002 to 0.003 mm., and 

 under tension, it may be much less than 0.01 sec., while with a 

 silver wire having a diameter of about 0.02 mm., it may approach 

 0.1 sec. when the tension is relaxed. If the tension on the silvered 

 quartz fiber is very much relaxed, the instrument becomes 

 unduly sluggish and as much as 10 sec. may elapse before the 

 deflection is completed. The galvanometer is then exceedingly 

 sensitive but the zero reading is likely to be unsteady and the 

 fiber may move out of focus. 



The advantages of this form of galvanometer are its extreme 

 quickness of action and immunity from the effects of stray 

 fields. 



In a high-resistance circuit the damping is by air friction, but 

 if the resistance be low and shunts are employed electromagnetic 

 damping is also present. Einthoven has shown 10 that if a high 

 resistance instrument is placed in series with an adjustable 

 resistance and in parallel with a condenser and the whole com- 

 bination shunted around another resistance, it is possible to 

 adjust the combination so that the galvanometer is dead beat. 

 The instrument thus becomes a low-period oscillograph, suitable 

 for recording phenomena whose cycle is completed in a few 

 tenths of a second.* 



By using an arc lamp and the proper optical system, the 

 deflection may be projected on a screen at about a meter's dis- 



* The firm of Gans and Co. manufacture a regular oscillograph (without 

 damping) based on the "string" principle. 



