oi Hydrogen and some other Gases. 317 



A quantity of mercury then flows into the cylinder and 

 partially fills the bottom of it. In this state the cylinder is 

 transportable to the Cailletet compressor without any risk 

 of introducing other oases into it. 



(d) Electromagnet. 



The magnetic field was obtained by a Weiss electromagnet, 

 the pole-pieces being always 1 cm. apart. The end surface 

 of the pole-pieces was a circular section of 1 cm. in diameter. 

 The magnetizing current of 10 amperes produced a field of 

 22,000 gauss in the place where the magnetic measurement 

 was to be made. In the case of air 4 amperes were used 

 and the corresponding field 12,500 gauss. During the 

 magnetization a slight convection of air was produced by 

 the heating of the electromagnet, and this made the obser- 

 vation of the deflexion of the mirror somewhat difficult. 

 To avoid the disturbing effect the coil of the magnet with 



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the exception of the pole-pieces was entirely covered with a 

 winding of lead tubing and the water mantles, water being 

 constantly circulated through the lead tubing and mantles 

 during the observations. 



The intensity of the field was measured by means of an 

 exploring coil and a ballistic galvanometer as usual. 



(e) Method of Experiments. 



In the magnetic balance a delicate knife-edge rests on a 

 smooth steel plane, so that a very minute gradual displace- 

 ment of the knife-edge, either translational or rotational, 

 can never be absolutely avoided. This gradual displacement 

 is usually accelerated when the field was repeatedly applied, 

 causing the gradual displacement of the zero point on the 

 scale. However, by comparing the results obtained when 

 such a gradual displacement of the zero-point occurred and 

 when it was absent, it was found that this displacement did 

 not affect the final results, provided the mean of the successive 

 zero-points in each observation be taken as the true zero of 

 the deflexion. 



In making the observations, we first passed a current of 

 10 amperes in the electromagnet, and the maximum deflexion 

 of the scale was observed. It took usually 30 to 60 seconds. 

 Then the current was quickly reduced to zero, and the final 

 deflexion or the zero-point was observed. These processes 

 were repeated usually ten times, and the mean of these 

 deflexions was taken. The whole process required about 

 ten minutes. The temperature of the gas under examination 



