en. XT.] 



THE GALVANOMETER 



117 



wrong in supposing that the contact of dissimilar metals with tissues 

 proved his point. 



This conclusion has been arrived at by certain new methods of 

 investigation. In 1820 Oersted discovered electro-magnetism : that 

 is, when a galvanic current passes along a wire near a magnetic 

 needle, the needle is deflected one way or the other, according to 

 the direction of the current. This led to the invention of the 

 astatic needle and the galvanometer, an instrument by which very 

 weak electrical currents can be detected. For a long time the subject 

 of animal electricity, however, fell largely into disrepute, because of 

 the quackery that grew up around it. It is not entirely free from 

 this evil nowadays ; but the scientific investigation of the subject has 

 led to a considerable increase of knowledge, and among the names 

 of modern physiologists associated with it must be particularly 

 mentioned those of Du Bois Eeymond and Hermann. 



Before we can study these it is, however, necessary that we should 

 understand the instruments employed. 



The Galvanometer. The essential part of a galvanometer is a 

 magnetic needle suspended by a delicate thread ; a wire coils round 



Fio. 133. 



FIG. 134. 



it; and if a current flows through the wire, the needle is deflected. 

 Suppose a man to be swimming with the current with his face to the 

 needle, the north-seeking pole is turned to the left hand. But such a 

 simple instrument as that shown in fig. 133 would not detect the feeble 

 currents obtained from animal tissues. It is necessary to increase 

 the delicacy of the apparatus, and this is done in several ways. In 

 the first place, the needle must be rendered astatic, that is, independent 

 of the earth's magnetism. The simplest way of doing this is to fix 

 two needles together (as shown in fig. 134), the north pole of one 

 pointing the same way as the south pole of the other. The current 

 is led over one needle and then over the other ; the effect is to pro- 

 duce a deflection in each in the same direction, and so the sensitive- 

 ness of the instrument is doubled. If now the wire is coiled not only 

 once, but twice or more in the same position, each coil has its effect 



