16 INDUCED CUKEENT. [CH. .1 



discoloured when the current passes : the needle points 

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



(23) Induced cwrrent. 



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 mounte'd 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 current can be observed. The wires from the coil 

 are most conveniently connected by means of the insu- 

 lated 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 position of the coil 

 when the protoplasmic circulation has been stopped. 



