HALL. — CONDUCTIVITY OP MILD STEEL. 289 



Greater vigilance, and probably greater accuracy, marked the later 

 work. 



The difficulty here discussed can be very much reduced by a more 

 sparing use of gas flames in the room. 



The Observations. 



As an example of the observations in the main experiment, showing 

 their character and their arrangement, the last set made will now be 

 given, some unimportant details being omitted. 



The letters L and R refer merely to the position of the commuta- 

 tors. A change from L io R reverses the current in the galva- 

 nometer concerned. In each circuit the first observation is made 

 under Z, the second under R, the third under L, etc. 



The note " 1, 4, 7, 10," means that the junctions of the outer circle 

 of the disk (see Fig. 4), both above and below, are in use, connected 

 as in Figure 9. After this set of junctions, those of the next circle, 

 2, 5, 8, 11, are used, etc. 



Each group of seven or five observations in the first circuit was 

 made between two groups of three observations in the second circuit. 

 Before the first, third, fifth, etc., groups of three in the second circuit, 

 the main outlet of the water from the lower chamber of the apparatus 

 was closed for a few seconds to wash out, through the overflow tubes 

 (Fig. 1), the air-bubbles that might have accumulated beneath the 

 disk. This operation did not greatly affect the current in the secona 

 circuit. The course of operations gave the first circuit about two 

 minutes for recovery after the disturbance before observations in this 

 circuit were renewed. 



The water had been running through the apparatus for about an 

 hour, as usual, before the observations began. 



VOL. XXXI. (n. 8. XXIII.) 19 



