20 ELECTRIC SHOCK. [CH. I 



the region between the punctures is killed and becomes 

 discoloured when the current passes : the needle points 

 should not be more than 2 — 3 mm. apart. 



(23) Tetanising current 



Two triangles of platinum foil are sealing- waxed on to 

 a glass-slip, the points being about 1 mm. apart. To 

 make the platinum adhere well it is necessary to heat the 

 glass over a flame until the wax between the glass and 

 the metal is thoroughly soft, and then to apply pressure. 

 An Elodea leaf is mounted in water so that a cell, showing 

 circulation, lies between the points, and by connecting 

 the foil triangles with an induction coil, the effect of 

 the tetanising current can be observed. The wires from 

 the coil are most conveniently connected by means of 

 the insulated screw-binders, obtainable from instrument 

 makers ; in the absence of screw-binders the following 

 arrangement will be found to answer quite well. A cork 

 ring is sealing-waxed on to each foil-triangle near its 

 base, and into the little vessels so made, mercury is 

 poured, into which the connecting wires are placed. To 

 get a rough idea of the current needed, it is advisable 

 to note the position of the coil when the current is just 

 bearable on the tongue, and compare it with the position 

 of the coil when the protoplasmic circulation has been 

 stopped. 



