18 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



the simplified form used in this work. Only ordinary ingenuity is re- 

 quired for its construction, and the cost is comparatively small. 



The galvanometer. — Figure 5 shows the construction of the galva- 

 nometer. The magnetic field is furnished by eight cast-iron ring mag- 

 nets, arranged in two sections of four. One section is placed above the 

 focussing lenses 1, and the other section directly beneath this. A thin 

 brass chamber fitted between the jaws of the compound magnet carries 



the filament, a platinum fiber 

 about one ten-thousandths of an 

 inch in diameter. These three 

 pieces, with some simple device for 

 varying the tension on the fiber, 

 completes the galvanometer. 



Periodic flashes fo7- giving time 

 coordinates. — Figures 6 and 7 

 show in some detail the mechan- 

 ism employed for giving periodic 

 flashes. Two rachet wheels, each 

 carrying sixty teeth, are fastened 

 rigidly to an axle (Figure 7). A 

 double electromagnet m attracts 

 the armature a and causes the 

 clutch X to turn the axle. Clutch 

 y is so adjusted that the rotation 

 is allowed to proceed only a dis- 

 tance of one tooth. So long as 

 the current continues through m 

 the axle is held firmly in position ; but whenever this current is broken 

 the armature is pulled back by spring s so that clutch x falls behind 

 another tooth. Upon making the circuit again, the axle is again ro- 

 tated a distance of one tooth. A relay operated by a separate circuit 

 is so connected into the circuit which traverses the magnet m, that 

 when its armature is back the circuit is made through m. The relay is 

 operated by a circuit that is made for an instant once a second, a sec- 

 onds pendulum being used for accomplishing this. Thus the circuit m 

 is broken for an instant once a second, and the axle, therefore, is 

 rotated one division every second. 



A thin aluminium disk twenty centimeters in diameter is clamped 

 to the end of the axle by means of a thum-screw. Narrow slits are cut 

 radially into the edge of the disc as shown in Figure 7. This disc is 

 interposed between a Nernst glower and slit Si of the galvanometer. 

 Figure 4. An iron screen i before the glower allows only a narrow 



Fig. 5. 



