4 Dr. Fletcher on the Determination of the B.A. Unit 



preliminary experiments in which the wires were twisted 



together was finally traced to leakage, although the wires had 



a double covering of silk. Both circuits were provided with 



commutators. The sine-galvanometer had a horizontal bar 



parallel to the axis of the coil. To one end of this was 



attached a telescope, beneath which was a short scale which 



was seen by reflection in the mirror of the needle, and allowed 



a very accurate setting to be made without bringing the needle 



to rest. The needle consisted of two thin strips of steel 1*2 



centim. in length, separated by a piece of wood '6 centim. in 



thickness. The circle of the galvanometer was graduated to 



half-degrees, and read by verniers to one minute. 



The needle was acted upon by both currents simultaneously, 



and by means of the commutators the actions were caused to 



be in the same and in opposite directions alternately. The 



current through the sine-galvanometer is d in the formula 



cc'JUt 

 J= —, — . The current through the coil on the wooden circle 



is c + </, and was assumed equal to c, as c' was less than -00007c. 

 Let G denote the constant of the fixed coil, Gr' that of the 

 sine-galvanometer, H the horizontal magnetic force, 6 and 6 r 

 the deflections when the actions are in the same and in opposite 

 directions respectively. Then 



Hence 



Gccosfl + GV = Hsin0, 

 Gccos0'-GV=Hsin0'. 



H + irala\ < H sin 4(0-0') 



(d + 0') 



Let I denote the length of the wire in the fixed coil, and b the 

 distance of the needle from its plane. Then 



G= 



Hence the equation for J becomes 



4tt 2 G' h cos 1(0 + 0') 



I shall discuss in order the quantities contained in this 

 expression. 



R/, the resistance of the secondary circuit, is the sum of the 

 resistances of the 30,000-ohm coil, the sine-galvanometer, and 



