OF ARTS AND SCIENCES. 95 



safely assume that the character of the phenomena observed with the 

 former under the different conditions considered would be substantially 

 the same as those occurring with the latter and weaker current. 



The alternating current thus produced, which we will call the "line 

 current," was measured by an electro-dynamometer included in the 

 circuit, and placed between the resistance frame and the telephone 

 coil. This electro-dynamometer was constructed especially for the 

 purpose, and was calibrated in the ordinary manner, by passing a 

 direct current of known and variable strength through its coils, making 

 the usual reversals to eliminate the effect of the earth's magnetism. 



The intermittent illumination needed for the purposes of our experi- 

 ment was furnished by the sparks produced by a Helmholtz tuning- 

 fork interrupter, making 128 complete vibrations per second. At 

 each vibration, when the style broke contact with the mercury in the 

 cup a bright spark was produced. The fork was so placed that this 

 flash should illuminate the field of a microscope placed opposite, and 

 which was focussed upon the end of a style carried by the diaphragm 

 of the receiving telephone. The illumination given by the sparks, 

 especially when concentrated by a lens, was abundantly sufficient to 

 enable the observer to see the style as a silhouette against a bright 

 field. The telephone was so placed that the vibration of the style 

 was in a horizontal direction. Wiien no current passed through the 

 telephone the style was of course seen continuously, on account of the 

 rapid recurrence of the sparks. If the rate of alternation of the alter- 

 nating current employed is exactly 128 per second, thus coinciding in 

 frequency with the sparks, then when this current is sent through the 

 telephone coil, although the diaphragm will enter into corresponding 

 vibration at the same rate, yet the style carried by it will still seem 

 to be at rest when viewed by the microscope. But if the rates are 

 not exactly the same, then of course the familiar stroboscopic effect 

 will be produced, and the style will appear to be in a state of slow 

 vibration, so that the amplitude of the vibration, if sufficient in amount, 

 can readily be measured by means of a spider-line micrometer. 



Using an objective having a focal length of half an inch, and an 

 eye-piece of moderate power, and with a very weak magnet in the 

 telephone, we found no difficulty in producing a perfectly measur- 

 able vibration with a current of only 5 milliamperes, while with a 

 current of 21). 5 milliamperes the excursion rose to fourteen thou- 

 sandths of a millimeter. 



On passing a current through the magnetizing coil alone, the dia- 

 phragm is of course immediately drawn towards the core by a certain 



