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■ VII. A necessary Modification of Ohm's Law. 



By Feknando Sanfokd*. 



[Plate III.] 



DURING the past year I have been engaged in some 

 investigations which seem to call in question the validity 

 of Ohm's law, by showing that the resistance of a metallic 

 conductor varies with the nature of the dielectric in its field 

 of force. For the purpose of determining if this be the 

 case, I have made a large number of measurements of the 

 resistance of a copper wire in various dielectrics, and have 

 found that in several cases the change of resistance, both in 

 liquid and gaseous dielectrics, is very marked. 



The apparatus used consists of a copper tube about four 

 feet long and one inch in internal diameter, closed with 

 copper plates at the end, and having a copper wire 1 millim. 

 in diameter stretched through its centre and fastened by 

 means of a binding-screw to the centre of one end plate, 

 while it passes through an insulating-plug in the centre of the 

 other end plate. The tube is provided with stopcocks at the 

 ends for filling and emptying, and with an opening in the 

 side for inserting a thermometer. The current, which was 

 always between the extremes of five and eight niilliamperes, 

 was passed one way through the tube and back through the 

 wire, so that the whole dielectric in its fieid could be changed 

 at will. The measurements were made by means of a Wheat- 

 stone's bridge with arms of 1 : 1000, and a sensitive galvano- 

 meter. A change of resistance of O'l ohm in the box, which 

 corresponded to -0001 ohm in the tube and wire, produced 

 a very noticeable deflexion of the galvanometer-needle. The 

 measurements were accordingly estimated with a fair degree 

 of accuracy to '00001 ohm. 



The resistance of the tube and wire was measured in air at 

 different temperatures through a range of about ten degrees 

 Centigrade, and a curve was plotted, using the temperatures 

 as abscissse and the resistances as ordinates. This curve did 

 not depart appreciably from a straight line. The dielectric 

 to be tested was then poured into the tube, a set of measure- 

 ments was made in it through the same range of temperatures, 

 the dielectric drawn out, the tube cleaned and dried, and a 

 new set of measurements made in air. This was repeated 

 several times, until it was certain that after each change of 

 dielectrics the resistance returned to the same value which it' 

 had previously shown in the same dielectric. In the case of 

 air and petroleum this comparison was carried on for a month, 

 the dielectrics in the meantime being changed five times, ami 

 * Communicated by the Author. 



Phil. Mag. S. 5. Vol. 35. No. 212. Jan. 1893. F 



