364 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Strength of the Magnetic Field. 



The magnetic field was obtained between the poles of a large 

 electromagnet. The pole-pieces were plane and parallel, about 4.2 cm. 

 in diameter and 0.5 cm. from each other. For measuring the strength 

 of the magnetic field, a coil consisting of a single turn of fine wire was 

 jerked out of the field by means of a spring, which was released by means 

 of a long cord pulled by the observer at a ballistic galvanometer. The 

 coil was jerked quickly by the spring to a distance of 34 cm. from the 



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Figure 4. 



pole-pieces. No appreciable effect was obtained by jerking it from this 

 position to a point 2 meters farther away. 



The ballistic galvanometer was situated 8 or 9 meters from the 

 electromagnet. The suspension of this instrument was made of hol- 

 low bell-shaped magnets with hard-rubber cores, to give considerable 

 moment of inertia with small damping effect. The mirror was made 

 of a narrow strip of heavy glass. No vane was placed in the suspen- 

 sion, which was supported by a fine quartz fibre. An earth-inductor, 

 in series with the one-turn test-coil mentioned above, was 10 or 11 

 meters from the electromagnet. It had been wound with six sets of 

 turns by Professor Hall, whose values for the areas of these turns 

 were taken. By making use of one or more of the six sets of turns 



