34 ADVANCED ELECTRICITY AND MAGNETISM. 



Note. Lord Rayleigh in his determination of the electrochem- 

 ical equivalent of silver* used an electrodynamometer of which 

 the essential features are shown in Fig. 30. A horizontal cir- 



balance 



B 



Fig. 30. 



cular coil B is suspended from one end of a balance arm and 

 pushed up or down by the electromagnetic action of two station- 

 ary coils A A. All three coils are connected in series, and the 

 value of the current is calculated from the observed push on B 

 and the sizes and relative positions of the coils. 



PROBLEMS. 



1. A circular coil of wire of 20 centimeters radius has 15 turns 

 of wire. How much current is required in the coil to produce at 

 the center of the coil a field intensity of 0.57 gauss? Ans. 0.121 

 abampere. 



2. A certain current gives a deflection of 42 on a tangent 

 galvanometer at a place where the horizontal component of the 

 earth's magnetic field has a value of 0.18 gauss. The diameter 

 of the coil of the galvanometer is 30 centimeters and the coil 

 contains 5 turns of wire. Find the value of the current in 

 abamperes. Ans. 0.0774 ampere. 



Note. Assume the section of the coil itself to be very small; that is to say, 

 take 30 centimeters as the mean diameter of the coil. 



3. A tangent galvanometer has two coils of wire A and B 

 placed side by side as shown in Fig. 3. The needle ns of the 



* See a paper on The Electrochemical Equivalent of silver by Lord Rayleigh 

 and Mrs. H. Sidgwick, Philosophical Transactions, 1884, part II, pages 411-460. 



