S. R. Williams — Electromagnetic Effect. 303 



reason why. As the tubes get smaller and smaller there is a 

 greater departure from a straight-line variation of the displace- 

 ment of the mercury with field strength. It is evident that 

 the friction of flow of the mercury in the tubes is the cause of 

 this variation from the straight-line law because the frictional 

 forces become larger the smaller the bore of the tube. In 

 curve 5 the frictional force has become so large that it reduces 

 the amount of the displacements for the different field strengths 

 to values lower than that in curve 4. One may conclude that 

 some tube with dimensions between those of •! and 5 would 

 give a maximum displacement for the field strengths used in 

 this work. If the frictional force is a constant, then all of the 

 curves shown in fig. 8 will become practically straight lines 

 beyond certain field strengths. The curves indicate this to be 

 true. 



The results obtained from these five tubes show that this is 

 a simple and accurate means for measuring magnetic field 

 strengths over wide ranges. No difficulty was experienced in 

 measuring fields as low as 50 gauss. In actual practice we cali- 

 brated the tubes with a solenoid whose field was known, and 

 then used the tube between the poles of the electromagnet 

 whose field was to be determined. This eliminated the diffi- 

 culty spoken of by those who have used Leduc's form of appa- 

 ratus, viz., that the thickness of the conducting sheet of mercury 

 must be known with great accuracy if one is to calculate the 

 field strength from the amount of displacement of mercury, the 

 current strength, and the dimensions of the tube. 



If one can do sufficient glass-blowing to put together glass 

 tubing as shown in fig. 6, an instrument can be easily and 

 cheaply made which, when calibrated, may be used for accu- 

 rately measuring unknown fields, and this without knowing 

 any of its dimensions. 



Summary. 



1. This paper has attempted to explain the mechanism of this 

 electromagnetic effect by assuming that in a liquid conductor 

 we have a bundle of conducting filaments. The current flow- 

 ing through these in a magnetic field will behave as it does in 

 a flexible conductor in a magnetic field. In any case, whether 

 we are dealing with conduction through gases, liquids, or 

 solids, the effect obtained is due to the reaction between the 

 magnetic field produced by the moving charges and the field 

 in which the charge is moving. 



2. This work has developed a simple and accurate method 

 for measuring magnetic field strengths. 



Physical Laboratory, 



Oberlin College, Oberlin, Ohio, 

 March, 1912. 



