493-495] Galvanometers 425 



so that we must have 



; = ^sin<9 (418), 



Or 



where G, the galvanometer constant, has the same meaning as before. 



This instrument has the disadvantage that it cannot be used to measure 



TT 



currents greater than -~ . It is, however, sensitive over the whole range 



through which it can be used : if dO is the increase in caused by a change 

 di in i, we have 



dO ^r sec ddi, 



12 



so that the greater the current the more sensitive the instrument. 



The great advantage of this form of galvanometer, however, is that when 

 the reading is taken the magnet is always in the same position relative 

 to the field set up by the current in the coil. Thus the deviations from 

 uniformity of intensity at the centre of the field do not produce any error 

 in the readings obtained : they result only in the galvanometer constant 

 having a value different from that which it has so far been supposed to 

 have. But when once the right value has been assigned to the constant G, 

 equation (418) will be true absolutely, no matter how large the movable 

 needle may be in comparison with the coil. 



Other galvanometers. 



495. There are various other types of galvanometers in use to serve 

 various purposes other than the exact measurement of a current. For full 

 descriptions of these the reader may be referred to books treating the 

 theory of electricity and magnetism from the more experimental side. 

 The following may be briefly mentioned here : 



I. The D'Arsonval Galvanometer. This instrument is typical of a class 

 of galvanometer in which there is no moving needle, the moving part being 

 the coil itself, which is free to turn in a strong magnetic field. The coil 

 is suspended by a torsion fibre between the poles of a powerful horseshoe 

 magnet. When a current is sent through the coil, the coil itself produces 

 the same field as a magnetic shell, and so tends to set itself across the 

 lines of force of the permanent magnet, this motion being resisted by no 

 forces except the torsion of the fibre. 



II. The Mirror Galvanometer. This is a galvanometer originally designed 

 by Lord Kelvin for the measurement of the small currents used in the trans- 

 mission of signals by submarine cables. The design is, in its main outlines, 

 identical with that of the tangent galvanometer, but, to make the instrument 

 as sensitive as possible, the coil is made of a great number of turns of fine 



