HALL. — CONDUCTIVITY OF SOFT IRON. 133 



spiral beyond it was perhaps 2 cm. The velocity of the stream between 

 the bulb and the spiral was some 30 or 40 cm. per second. One stream 

 was usually about 6°. 7 warmer than the other. The method of alterna- 

 tion was used ; that is, if the stream passing spiral No. 1 was during one 

 set of observations kept a certain number of degrees warmer than the 

 other stream, the streams were exchanged at the end of that set and an- 

 other set was then made, the difference uf temperature and the mean tem- 

 perature remaining nearly as before, and no part of the apparatus suffering 

 change of place except the cock by means of which the change of flow 

 was affected. The combination of the two complementary sets of obser- 

 vations gave a result from which errors due to disagreements of the ther- 

 mometers and lack of perfect equality of the spirals was practically elim- 

 inated. Sets of observations were made at various mean temperatures, 

 and, in order to make the results at these various temperatures com- 

 parable, slight corrections, amounting at the most to less than one-half 

 of one per cent, were made in certain cases because of errors in the 

 graduation of the thermometers, the combined diameters of the bores 

 being slightly greater at some temperatures than at others. Similar cor- 

 rections made in calibrating the copper-iron thermo-electric junctions 

 have already been referred to. 



Each of the horizontal lines, numbered from 1 to 8 in the table below, 

 gives the result of two complementary sets of observations, such as have 

 been described above. T is the mean temperature of each pair of sets ; A is 

 the mean difference of temperature of the streams ; L is the mean length 

 of bridge wire included between the two points of equilibrium correspond- 

 ing to the two positions of the commutator of the Carey Foster bridge. 



May 



The mean resistance of the bridge wire used in this test was 0.001491 

 ohm per cm., the material being German silver and the diameter about 

 0.2 cm. The mean resistance of the bridge wire, German silver about 

 0.4 cm. in diameter, employed in later experiments, when the spirals 



