142 THE ELECTRICAL PHENOMENA OF MUSCLE. [CH. xii. 



face to the needle, the north pole is turned to the left hand. 

 But such a simple instrument as that shown in fig. 160 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. 161), 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 produce a 

 deflection in each in the same direction, and so the sensitiveness 

 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 on the needles ; the multiplication of the effect of a 

 weak current in this way is accomplished in actual galvanometers 

 by many hundreds of turns of fine wire. 



Fig. 162 illustrates the best galvanometer : that of Sir 

 William Thomson (now Lord Kelvin). Jt is called a reflecting 

 galvanometer, because the observer does not actually watch the 

 moving needle, but a spot of light reflected from a little mirror, 

 which is attached to and moves with the needle. A very small 

 movement of the needle is rendered evident, because the move- 

 ment of the spot of light being, as it were, at the end of a long 

 lever namely, the beam of light, magnifies it. 



Non-polarisable Electrodes. If a galvanometer is connected 

 with a muscle by wires which touch the muscle, electrical currents 

 are obtained in the circuit which are set up by the contact of 

 metal with muscle. The currents so obtained form no evidence 

 of electro-motive force in the muscle itself. It is therefore 

 necessary that the wires from the galvanometer should have 

 interposed between them and the muscle some form of electrodes 



which are non-polarisable. Fig. 163 

 shows one of the earliest non-polarisable 

 electrodes of Du Bois Reymond. It 

 consists of a zinc trough on a" vulcanite 

 base. The inner surface of the trough 

 is amalgamated and nearly filled with 

 a saturated solution of zinc sulphate. 

 In the trough is placed a cushion of 

 blotting-paper, which projects over the 

 Fig. i63.-Non-poiansabie elec- edge of the trough ; on it there is a 

 (M^KendiSk.? 018 E * ymond - pad of china clay or kaolin, moistened 

 with physiological salt solution (o'8 per 



cent. NaCl) ; on this pad one end of the muscle rests. The binding 

 screw (k) connects the instrument to the galvanometer ; the other 



