REFLECTING GALVANOMETER AND SHUNT. 



547 



ammonium chloride, while the porous cell contains compressed carbon in a fluid mixture of 

 black oxide of manganese and carbon. It is most frequently used for electric bells, as its feeble 

 current lasts for a long time. 



The carbon is the + pole, and the zinc the - .] 

 Non-polarisable Electrodes. If a constant current be applied to moist animal tissues, e.g., 



Fig. 386. 

 Z, zinc ; H, movable support ; C, 

 386. Brush electrodes of v. Fleischl. 



Fig. 385 

 Fig. 385. Non-polarisable electrode of du Bois-Reymond 

 clay point the whole on a universal joint. Fig. 



nerve or muscle, by means of ordinary electrodes composed either of copper or platinum, of 

 course electrolysis must occur, and in consequence thereof 

 polarisation takes place. In order to avoid this, non-polaris- 

 able electrodes (figs. 380, 385) are used. Such electrodes are 

 made by taking two pieces of carefully amalgamated, pure, 

 zinc wire (z, z), and dipping these in a saturated solution of 

 zinc sulphate contained in tubes (a, a), whose lower ends are 

 closed by means of modeller's clay (t, t), moistened with "6 

 per cent, normal saline solution. The contact of the tissues 

 with these electrodes does not give rise to polarity. [The 

 brush electrodes of v. Fleischl are very serviceable (fig. 386). 

 The lower end of the glass tube is plugged with a camel-hair 

 pencil, moistened with modeller's clay, otherwise the arrange- 

 ment is the same as shown in fig. 380, IV.] 



Arrangement for the Muscle- or Nerve-Current. In order 

 to investigate the electrical currents of nerve or muscle, the 

 tissue must be placed on non-polarisable electrodes, which may 

 either be one of the forms described above, or the original form 

 used by du Bois-Reymond (fig. 380). The last consists of two 

 zinc troughs (p, p) thoroughly amalgamated inside, insulated on 

 vulcanite, and filled with a saturated solution of zinc sulphate 

 {s, s). In each trough is placed a thick pad or cushion of 

 white blotting paper (b, b) saturated with the same fluid [de- 

 riving cushions]. [The cushion consists of many layers, almost 

 sufficient to fill the trough, and they are kept together by a 

 thread. To prevent the action of the zinc sulphate upon the 

 tissue, each cushion is covered with a thin layer of modeller's 

 elay (t, t), moistened with 0'6 per cent, saline solution, which 

 is a good conductor [clay guard]. The clay guard prevents 

 the action of the solution upon the tissue. Connected with 

 the electrodes are a pair of binding screws, whereby the appa- 

 ratus is connected with the galvanometer (fig. 380). 



[Reflecting Galvanometer. The form of galvanometer, 

 used in this country for physiological purposes, is that of Sir 

 William Thomson (fig. 387). In Germany, Wiedemann's form 

 is more commonly used. In Thomson's instrument, the 

 astatic needles are very light, and connected to each other Thomson s 

 by a piece of aluminium, and each set of needles is surrounded 

 by a separate coil of wire, the lower coil (I) winding in a 

 direction opposite to that of the upper (u). A small, round, 

 light, slightly concave mirror is fixed to the upper set of needles. The needles are sus- 

 pended by a delicate silk fibril, and they can be raised or lowered as required by means of a 



Fig. 387. 



reflecting galvano- 

 meter, u, upper, I, lower coil ; 

 s, s, levelling screws ; m, mag- 

 net, on a brass support, b. 



