PAPER BY PROF. HAGEN. 



9 



bevelled ou the sides that cut through the air, rest upon a vertical metal 

 axis which communicates the rotary motion to them. Each of these 

 arms is 8 feet or 96 Rhenish inches long and ou its end the disk is fas- 

 tened whose resistance is to be measured. In order lo prevent the 

 bending of the arms they are held not far from their ends by small 



^ff.2. 



wires which pass over a sup- 

 })ort 18 inches high vertically 

 above the vertical axis. The 

 drawing presents only the 

 connection of the two arms 

 between themselves and with 

 the axis. The latter is in its 

 upper portion turned slightly 

 conical and carries the corres- 

 ponding hollow hub which is 

 sciewed to the brass plate 

 under the arms. 



The rotation is brought 

 about by the tension of two 

 small threads which are wound 

 in the same direction around 

 the ivory spindle that is fas- 

 tened to the axis, and are then 

 drawn in opposite directions 

 over two rollers and drawn 

 taut by light scale pans with 

 weights therein. These rollers 

 I had formerly fastened at the 

 greatest iiossible distance on 

 the opposite walls of the room 

 in order that when winding 

 up the weights the threads 

 might lie uniformly alongside 

 of and not over each other, 

 but this design was by no 

 means certainly attained and 

 the far-stretched threads ma- 

 terially increased the labor 

 of the observation, especially 

 since the arms and the disks 

 fastened to them occasionally came in contact with these threads. 



When in the past summer I again undertook the observations I placed 

 the rollers, as the drawing shows, close to the axis, but did not let 

 the latter stand upon a fixed point, but rather ijrovided it with a 

 screw thread on its lower part whose mother is cut into a thick 

 plate of brass. By rotation the axis therefore rose or sank uniformly. 



