54 



SCIENCE 



[N. S. Vol. XXVI. No. 654 



perhaps the most common one. Ampere's laws 

 give as the equation for the force, F, acting 

 on a eondnctor of length I centimeters lying 

 at right angles to a field of intensity f, 



F = ilf. 



ployed. To avoid these difficulties, a field is 

 obtained by using a C-shaped cast-iron cylin- 

 der as the core of an electromagnet. The 

 length of the cylinder used is 9.5 cm.; its in- 

 side diameter, Y.7 em. ; and its outside 

 diameter, 11.4 cm. The width of the air gap 



Fig. 1. 



The lack of any simple means of measuring 

 directly the force acting on the conductor in 

 this case led to the construction of the ap- 

 paratus herein described. 



The apparatus also serves well to illustrate 

 the basis on which we construct our definition 

 of the absolute electromagnetic unit of cur- 

 rent. 



On account of the smallness of the strength 

 of the earth's field, either a very large cur- 

 rent must be used with it, to produce an effect 

 of convenient magnitude to measure, or else 

 delicate means of measurement must be em- 



is 4.0 mm. The magnetizing coil has 400 

 turns of number 20 copper wire. 



The method of mounting the electromagnet 

 and conductor is shown in Fig. 1. The con- 

 ductor AB, which lies in the air gap is bent 

 up, then back, and clamped in two binding 

 posts, C and D, through which steel pins are 

 driven. The lower ends of these posts dip 

 into mercury cups. The steel pins form a 

 free axis of rotation. 



The force acting on the conductor is meas- 

 ured by the elongation produced in the spring 

 of a balance of the Linebarger type. In 



