950 



Transactions of the American Institute. 



Another test, made by putting two galvanometers (No. 18 and No. 

 9, coil 3) into same circuit, gave the following results : 



The near uniformity of these quotients gives evidence of very true 

 tangent galvanometers. 



12. The theorem, "The intensity of currents, as measured by the 

 tangent galvanometer, is proportional to the tangents of the angles 

 of deflection" may be verified in the following manner : 



Call the terrestrial magnetism, . whose tendency is to direct the gal- 

 Ta^s manometer needle to the magnetic 

 & meridian, the unit of directive force, 

 and let this unit be represented geo- 

 metrically by the line A M (Fig. 1), 

 which is the radius of the circle 

 M B M — the line MAM represent- 

 ing the meridian. Now, let an elec- 

 tric current be sent through the gal- 

 vanometer coil whose directive force 

 is precisely equal to the terrestrial 

 force, and whose tendency is to direct 

 the needle in a line perpendicular to 

 the meridian, and let this force be 

 represented by the A B. 



If the terrestrial force could now, 

 for a moment, be suspended, the nee- 

 dle would point due east and west, 

 '^but the combined action of the two 

 \M equal forces will direct the needle 

 toward the point of intersection of the line drawn perpendicularly 

 from M, and that drawn horizontally from B at 1, which direction 

 cuts the quadrant at 45°, the line M 1 being the tangent of 45°, 

 which is 1. 



Now, if we augment the intensity of the current through the coil 



