TANGENT GALVANOMETER. 487 



In order to increase the sensitiveness of the galvanometer that is, 

 the deflection 8 for a given current we must increase the value of G, 

 and diminish that of H. The value of G is increased by increasing 

 the number of turns by Schweigger's method, and by placing them as 

 near the needle as possible. In order to diminish H, a magnet is 

 placed at a certain distance, which produces at the centre of the 

 frame a magnetic field parallel, and in the opposite direction to, that 

 of the Earth. 



Use is sometimes made of a quasi-astatic system of two needles 

 (299), one of which is inside the frame and the other is outside ; 

 the action of the Earth on the movable system is then far feebler 

 without there being any appreciable modification in the action of the 

 current, which is exerted more particularly on the inner needle. We 

 may also use two frames, each having one of its needles in the centre, 

 and pass the current in opposite directions, so that the actions exerted 

 on the two needles are concordant. 



504. TANGENT GALVANOMETER. In order to determine the 

 absolute value of the strength of a current, besides knowing the 

 component H of the terrestrial magnetism, we must also know the 

 constant G of the galvanometer. The name tangent galvanometer \s> 

 given to a galvanometer, the wire of which has been coiled in such a 

 manner that this coefficient may be calculated from the dimensions 

 of the wire and the shape of the frame. 



If on the frame a wire L is coiled on a circle of radius a in such 

 a manner as to make n turns, and if the needle, which is supposed to 

 be infinitely small, is placed at a point of the axis at a distance u 

 from the circumference, we shall have (493) 



2TTa 2 La A7T 2 ? 



G = n = = - 



u* fc 3 L 



which gives 



I = --r- tan 8 = / - } tan 8 . 



La 



The distance u is equal to a, when the needle is at the centre 

 of the circle. 



If the length is to be taken into account, we must estimate the 

 strength of the field outside the axis of the currents by the formulas 

 of (368). 



The formula of the tangent galvanometer would be exact and 

 independent of the length of the needle if the field of the current 

 were uniform. This would be the case, for instance, with a 



