500 Mr. S. Bidwell on the Electrical Resistance 



to be tested is suspended at its middle point, P, from a support 

 10'5 metres above a metre-bridge, to the terminals, T T', of 

 which the ends of the wire are connected. Another wire, 0, 

 is soldered at one end to P, and connected through the gal- 

 vanometer, G, with the slider, S. A resistance of 100 ohms 

 is inserted in each of the gaps, R E/, and a commutator, K, is 

 interposed between the two-cell battery, D, and the bridge. 



With this arrangement, supposing that the two halves of 

 the wire A B are of uniform sectional area and in the same 

 physical condition, and that the various parts of the apparatus 

 are in fair order and adjustment, there will be a balance when 

 the slider is near the middle division of the scale. And if 

 the resistances in the circuit are independent of the direction 

 of the current, it is clear that the balance will be maintained 

 notwithstanding that the commutator K be reversed. But 

 this is found not to be the case. 



A series of experiments was made with a copper wire 

 •4 millim. in diameter (No. 28 B.W.G.), and having a total 

 resistance of 2'11 ohms. The commutator was first set so 

 that the current through the wire passed up the portion B 

 and down the portion A (i. e. in the direction B PA), and a 

 balance was obtained by adjusting the slider. The commu- 

 tator was then reversed and the current made to pass up A 

 and down B. This at once destroyed the balance, and in 

 order to restore it, it was necessary to move the slider several 

 divisions towards the right. Assuming that the total resist- 

 ance of the wire remains constant, this result may be explained 

 by supposing that the reversal of the current is accompanied 

 by increased resistance in the portion A, and diminished re- 

 sistance in the portion B. Owing to its vertical suspension, 

 the resistance of that portion of the wire in which the current 

 travels upwards is greater than it would be if the wire were 

 placed in a horizontal position, while the resistance of the 

 portion in which the current travels downwards is less. 



The experiment was repeated with an iron wire of larger 

 size, its diameter being *8 millim. (No. 22 B.W.G.). With 

 this the effect of reversal was smaller ; but it was well marked, 

 and of the opposite nature to that observed in the former case. 

 The readings obtained in the two series of experiments are 

 given in the following Table : — 



