Forces by means of Hydrostatic Pressure, 453 



pole- surfaces of 30 millim. in the side and 6 millim. distance 

 apart, I have even observed a magnetic rise of 95 millim., which 

 would correspond to a magnetic force of 20,000 C.ft.S. units. 

 I am indebted to Herr F. Braun for the use of the apparatus. 



In Table 1. 1 have given the magnetic rise in the magnetic 

 field of the Berlin electromagnet, between the pole-snrfaces of 

 24 millim. diameter and 36 millim. distance apart, when excited 

 by three, two, or one Bunsen's elements, giving; respectively a 

 magnetic force of* 12510, 10700, and 6383 C.Gr.S. units in the 

 magnetic field. 



The temperature of the room was maintained as nearly 

 constant as possible, and varied between 18° and 20° 0. 



The constant f is equal to the difference in the diamagnetic 

 constants of the fluid and atmospheric air divided by 87r. 

 The numbers in the column headed f . 10 10 give the increase 

 in pressure in gramme- per square centimetre of the bounding 

 surface between the fluid and atmospheric air in the interior 

 of the fluid, when the narrow limb of the U-shaped tube with 

 this bounding surface is placed in a magnetic field of in- 

 tensity 100,000 C.G.S. units. The last column gives the 

 arithmetic means of the values found for f for the different 

 magnetic forces. These differ with the concentration of the 

 solution and the nature of the substance dissolved. 



We may also observe A, and from that, by means of 

 equation 3 and the values given in Table I. for the quantity 

 f, we may calculate the value of the magnetic force Hi of 

 the magnetic held. Variations in the magnetic force, due to 

 variations in the strength of current furnished by a dynamo- 

 electric machine to excite the electro-magnet, become visible 

 by momentary variations in the meniscus of the magnetic 

 fluid. This method of observation might therefore be useful 

 in determining the intensity of a magnetic field. 



If the greatest possible accuracy is not required, the 

 magnetic fluid might, for such measurements, be contained in 

 a long narrow horizontal glass trough, and this be placed 

 partly within and partly outside of the magnetic field. A 

 strip of black paper may then be plunged vertically in the 

 glass trough, so that the curve formed by the surface of the 

 fluid is marked on the paper, from which, making allowance 

 for capillarity, the intensity of the magnetic field may be 

 roughly determined. 



The same method may be employed with gases, by re- 

 placing atmospheric air above a diamagnetic fluid by other 

 gases. I reserve the results of this investigation, with which 

 J. have been for some time engaged, for a later communi- 

 cation. 



