46 A. M. Mayer — Experimental Proof of Ohm's Law. 



Surely if an experiment, that is easily made, shows the 

 truth of a law of such theoretical and practical importance 

 as that of Ohm, even if it is one restricted in its range of C, 



E 



E and R, but shows within its limitations the relations C = — , 



iv 



then it should be made by all teachers of Physics so that clear 



physical conceptions of those relations may be given to 



students. As those who have seen these experiments have 



deemed them worthy of being more generally known, I now 



publish an account of them. 



.r o 



In the diagram the parts of the apparatus are shown, but 

 not at their relative distances apart or in the proper propor- 

 tions as to size. G is a low-resistance Thomson-galvanometer. 

 At L is the condensing lens of a lime-light lantern, which is 

 covered with a cap having a rectangular opening in it. Across 

 the middle of this slit is a vertical wire. The scale of the 

 galvanometer is at C, distance 165 cms from the mirror of the 

 galvanometer. The width of the divisions on this scale are 

 2'5 cms , and the lines are drawn 2 - 5 mms in breadth, or j 1 ^ the 

 distance apart of the centers of the lines forming a unit of the 

 scale. The scale is at such distance from the galvanometer- 

 mirror that the image of the vertical slit just fits in the space 

 of a scale unit, while the breadth of the image of the vertical 

 wire is exactly equal to the breadth of a scale line. This 

 arrangement gives the means of observing a deflection of the 

 beam of light to t 'q- and ^ of a unit with quickness and 

 accuracy. 



The image of the slit is so bright and that of the wire so 

 distinct that this method of observing deflections of the 

 galvanometer may be used in broad day light and the deflec- 

 tions may be read throughout the room. 



