BY DR. MICHAEL FOSTER. 353 



platinum foil, may be advantageously substituted for the 

 terminal portions of the copper wires. 



VI. Non-Polarizable Electrodes. In many cases, how- 

 ever, it will be absolute^ necessary to have non-polarizable 

 electrodes. The most convenient form is that of Du Bois 

 Reymond, modified by Bonders (fig. 272). 



A glass tube a (about one-third inch diameter is the most 

 convenient size) is plugged at one end by a plug b of china 

 clay, worked into a firm putty with .75 p. c. sol. of Na. Cl. 

 A few drops of a saturated solution of sulphate of zinc c are 

 carefully introduced into the tube. A slip of zinc, or piece of 

 zinc wire z, thoroughly amalgamated at the tip but covered 

 with varnish over the greater part of its length, is introduced 

 into the tube, and so placed that the amalgamated end dips 

 into the zinc solution as far as two or three millimetres above 

 the clay plug. The other upper end of the wire is bent round 

 the upper open end of the tube, and brought to the binding 

 screw of the brass collar d, which is movable up and down the 

 outside of the tube, and can be clamped at will. The copper 

 wire e is fastened in the same binding screw. 



Several such electrodes of different forms should be pre- 

 pared. The tube may be cut off straight at the lower end, 

 and the clay plug brought out in the form of a cone (fig. 273 

 A), or in any other shape that may be desirable. It is often 

 convenient that the end of the tube should be cut obliquely, 

 with the clay plug not projecting at all (fig. 273 c). The end 

 of the tube may be of the same diameter as the rest of the 

 tube, or may be brought more or less to a point Where the 

 electrodes require to be applied to nerves, it is convenient to 

 have the form fig. 273 B ; the end of the tube is bent round, 

 and the extreme point closed ; near the end, on the upper 

 surface, a small hole is drilled ; consequently the plug b is only 

 exposed at b f . 



Electrodes of different lengths should be prepared ; those 

 required for working in the moist chamber need not be more 

 than two inches long ; otherwise, five or six inches is a con- 

 venient length. 



The most convenient electrode-bearer is represented in fig. 

 272. The piece of leaden wire k ends in the brass head h r , 

 which bears the two arms f /, each of which holds an elec- 

 trode tube by means of a spring collar. The two arms move 

 round the point /i, and can be clamped in any position. The 

 points of the electrodes may thus be brought near to or apart 

 from each other, as may be desired. The extremely flexible 

 but non-elastic leaden wire (a cylindrical wire being far better 

 than a flat piece of lead), the far end of which is fixed in a 

 clamp, permits the pair of electrodes to be placed without re- 

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