PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 447 



been deflected, which force is the product of three factors, th« 

 intensity of terrestrial magnetism, the magnetic force of the 

 needle, and the line of the angle of deflection. The form of the 

 curve which represents the force with which the electrical currents 

 tend to deflect the needle is determined experimentally. Apart 

 from the geometrical construction, the rule may be thus expressed: 

 To measure the force of a current which is greater or less than 

 the assumed unit, observe, first, the deflection produced by it 

 when the coils lie in the magnetic meridian ; then turn the coils 

 in the former case backward, and in the latter case forward, until 

 the angle between the needle and the coils is equal to this angle 

 of deflection. The sine of this angle of deflection, divided b}' the 

 sine of the angle of deflection, produced after the rotation of the 

 coils, is the ratio of the current towards the assumed unit. 



The whole solution of the problem of Poggendorf, requires 

 several printed pages for its proper explanation. It is more in- 

 tricate, yet, doubtless, a nearer approximation to truth, than any 

 other method yet presented. Our associate. Dr. Bradley, who is 

 distinguished as a practical electrician, proposes to explain what 

 seems to him a far simpler and more practicable method of making 

 the galvanometer answer all questions regarding the intensity of 

 the current which deflects its needle, and we now shall have the 

 pleasure of listening to him. 



On the Anthistometer. — By Dr. L. Bradley, of New York. 



The Rheostat is an instrument for fixing the state and regulating 

 the flow of a current of electricity. It has been emplo^^ed under 

 diflerent forms by philosophers and professors, to prove theories 

 and establish principles. I have enlarged its field of usefulness 

 in making it a thing of every day practical utility in business. 

 The improved Kheostat and the improved Tangent Galvanometer 

 here exhibited, taken together, constitute an instrument for con- 

 veniently determining and correctly measuring the resistance 

 which conductors of electricity oppose to the free propagation 

 and transmission of a current through them, and the resistance of 

 coils, magnets, batteries, &c. It is a measure of resistance to 

 which I huve appled the name Anthistometer, from tlie Greek, 

 signifying a measure of resistance. 



This instrument I have in constant use; and in my business, in 

 relation to telegraphy, I have made it what the scale-beam or 

 yard-stick is in commerce. I have adopted the practice of mea- 



