162 PHYSIOLOGY OF MUSCLES AND NERVES. 



needles in opposite directions, the foi'ce of attraction 

 of the earth-magnetism is entirely, or almost entirely 

 removed, so that even very weak electric currents, if 

 caused to pass round the needle in a suitable way, can 

 cause a noticeable deflection of the needle. 



Fig. 37 represents a sensitive multiplier of a form 

 well suited for physiological experiments. The two 

 needles are connected together, and are suspended by 

 means of a thread of silk from the frame h' 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- 

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

 on to the frame (7; the binding screws /' / serve to 

 transmit the current. 



The use of tlie multiplier for physiological purposes 

 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, the strength 

 of the currents may be regarded as proportionate to the 

 trigonometrical tangents of the angle of deflection.^ In 

 order to measure slight deflections of this sort, our 

 former method of observation by means of the mirror 

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

 the magnet is in itself reflecting, or it is connected 

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

 copper sheath, A, which is closed by plates of looking- 



' See Notes and Additions, No. 9. 



