A. M. Mayer — Experimental Proof of Ohm's Law. 47 



An incandescent electric lamp with a part of its surface 

 (behind the plane of its filament) silvered may replace the 

 lime-light. Thanks to this arrangement, I have been able 

 during many years to make before my class electrical measure- 

 ments, and to measure the radiation, reflection, refraction, 

 diathermancy and polarization of radiant heat. 



At M is a magnet 25 cms long and 1-J cms in diameter. On this 

 magnet slides a wooden disc. At R is box containing 1, 2 and 

 3 Ohms of resistance, made of coils of copper wire. 



An insulated copper wire wound at its middle in a circle of 

 one coil, or in a spiral of any number of coils is placed over the 

 magnet and rests on the top of the wooden disc. The figure 



shows, (one-half size) how this circle of one coil is made. It 

 is bent around a wooden cylinder 3-J cms in diameter, and then 

 the free ends of wire are bent one half turn on each other. 

 The free lengths of the wire are then lashed to a light square 

 rod of wood as shown in figure. The wire and rod are then 

 coated with shellac to cement them firmly together. Rings of 

 spirals of 2, 3, 4, 5 and 6 coils are also made in the same 

 manner, but the coils are in a spiral, i. e. in one plane, and are 

 then cemented together with shellac between rings of thin 

 card-board. 



The length of wire forming each of these rings of spiral 

 coils with the portion on its handle is one meter long. 



The resistance of this length of wire added to the resistance 

 of the lengths between it and G and R, together with the 

 resistance of the galvanometer is (for convenience) made one 

 ohm. 



It may be well here to speak of the adjustment of the gal- 

 vanometer before describing the experiments, for I have 

 noticed in some laboratories -and lecture rooms galvanometers 

 which are used not as they should be. I have noticed that 

 the damping-magnet formed a considerable angle with the 

 plane of the coil. This was either because the median plane 

 of the coil was not in the magnetic meridian or because there 

 was considerable torsion in the suspending thread. 



In these galvanometers, or, at least, in mine, the median 

 plane of the coil is placed parallel to the faces of the drum of 

 the instrument. The plane of one of these faces is brought in 

 the magnetic meridian of the room, which has been carefulty 



