lf>2 PHYSIOLOGY OF MrsCI.KS AND NKKVI 3. 



needles iu opposite direct inns, the force of :it tract ion 

 of the earth-magnetism is entirely, or almosi entirely 

 removed, ><> that even very weak electric currents, if 

 cause, 1 to jnss round the needle in a suitable way. can 

 cause a noticeable deflection of the needle. 



Fig. :>7 represents a sensitive multiplier of a form 

 well suited for physiological experiments. The t\vo 

 needle- are connected together, and are suspended by 

 means of a thread of silk from the frame k' h; the screw 

 i serves to raise the needles to a proper height, so that 

 one of them can move- freely within the coils of the wire, 

 the other above the latter and over a graduated circle, by 

 which the deflection effected by the current can be mea- 

 Mired. The very thin wire, enclosed in silk, is wound 

 on to the frame C ; the hindm-' screws f f serve to 



I/ / 



transmit the current. 



The use of the multiplier for physiological pnrpo.-es 

 has recently considerably decreased, owing to the more 

 perfect adaptation of another form of apparatus, called 

 the tangent galvanometer, for such purposes. The ad- 

 vantage of this consists in the fact that it is not only 

 very sensitive, but it also allows the strength of the 

 current to be measured. If, for example, the deflec- 

 tions of the magnetic needle are very slight, t he strength 

 of the currents may be regarded as proportionate to the 

 trigonometrical tangents of the angle of deflect ion. 1 In 

 order to measure slight deflect ions of this sort, our 

 former method of observation hv means of the mirror 

 and lens may be used (chap, iv., 3, p. 57). Kither 

 the magnel [a in it.-e|f reflecting, or it is connected 

 with a mirror, and is suspended by a silk thread in a 

 copper sheath, J. which is closed by plates of looking- 

 1 Sec N'f i-s aii'l .\<l<lit ions, N.I. It. 



