( 109 ) 



The first photogram, fig. 1 j)latc 1 represents the deflections of the 

 string when currents of 1,2 and 3X^0 -^ Amp. are successively passed 

 tiu-ough the galvanometer. In the coordinate system a length of 1 mm. 

 of the abscissae has a value of 0.1 second, an ordinate of 1 mm. 

 representing 10 '^^ Amp. Although the image of the string has 

 considerable breadth and has no perfectly sharp outlines — as must 

 be expected with a magnification of (560 times — yet its displacement 

 in the coordinate system can easily be determined with an accuracy 

 of 0.1 mm. For if one of the margins of the image before and after 

 the deflection is observed, observation with the unaided eye or with 

 a magnify ing-glass will show that the deflection differs from the 

 tabulated amount by less than 0.1 mm. Hence the currents are 

 measured in the photogram with an accuracy of 10 ~^^ Amp. 



One notices that the deflections are accurately proportional to the 

 intensity of the current, that they are dead-beat and that they are 

 accomplished in 1 to 2 seconds according to their magnitude. The 

 strong damping must be ascribed to the resistance of the air, for 

 during the registering of the curves a resistance of one Megohm was 

 put into the galvanometer circuit by which the ordinary electromag- 

 netic damping was almost entirely suppressed. 



If tlie tension of the string is made ten times less, the galvano- 

 meter becomes ten times more sensitive and, as stated above, currents 

 of J0~'2 Amp. may still be observed. But with this greater sensi- 

 tiveness the deflections are no longer proportional to the current and 

 the movements of the string are difficult to record, as the quartz 

 thread no longer moves exactly in a plane. Yet the instrument can 

 still be used then for direct observation with the microscope. 



Figure 2 plate I shows that the deflections to the right and to 

 the left — in the figure corresponding to upward and downward 

 deflections, are equal. The velocity of the sensitive plate has 

 remained the same so that again an abscissa of one millimetre 

 corresponds to a time of 0.1 second. But the tension of the string 

 is 200 tim«s stronger so that one millimeti-e of the ordinates repre- 

 sents 2X10~^ Amp. A current of 4XI0~^ Amp. is alternately sent in 

 opi)osite directions through the galvanometer and hence causes 

 deviations of 20 mm. to the right and also to the left. It is easy to 

 ascertain that these deviations are equal to each other up to 0.1 millimetre. 



The movement of the string is very quick so that during the 

 deflection the string can only cast a feeble shadow on the sensitive 

 plate. The ascending and descending nearly vertical lines which in 

 the original negative are still visible as very thin streaks have become 

 invisible in the reproduced photogram. 



