612 White — Switch for Delicate Measurements. 



The method adopted involved less change in an instru- 

 ment already constructed, and is perhaps as good in any 

 case. The bus bar, raised by the inclined plane in its 

 backward travel, remains up as it returns to the fixed 

 frame (to secure this the spring was shifted so as to exert 

 a lifting action), and is then pulled down by a bell 

 crank, operated by the further return of the sliding 

 frame. This pulling down gives the rubbing. Motion 

 is transmitted to the crank by a whiffletree attached to 

 the two sliding frames, so that the return of either pulls 

 down the bus bar. The arrangement was constructed in 

 about 2 hours, mainly from strips of sheet brass. 



Eather thin bakelite of good insulating quality can now 

 be bought ready cut into strips, so that instead of fasten- 

 ing copper strips on the wooden bars by means of sheet 

 celluloid, it may perhaps be more advantageous to pin 

 them to bakelite which is fastened to the wood, provided, 

 of course, that there is no machining of the edges of the 

 strips. This construction is somewhat more robust, at 

 any rate. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C. 



