VAEIOUS FORMS OF KEYS. 



553 



as in the mercury key, which merely consists of a block of wood with a cup of mercury in its 

 centre. The ends of the wires from the battery dip into the mercury ; when both wires dip 

 into the mercury the circuit is made, and when one is out it is broken.] 



[Capillary Contact Key. "Where an ordinary mercury key is used to open and close the 



Fig. 395. 



Magneto-induction apparatus, with Stohrer's 



commutator. 



Fig. 396. 

 Du Bois-Reymond's friction key. 



primary "circuit, the layer of oxide formed on the surface by the opening spark disturbs the con- 

 duction after a short time ; hence, it is advisable to wash the surface of the mercury with a dilute 

 solution of alcohol and water ( W. Stirling). A handy form of " capillary contact " is shown in 

 fig. 398, such as was used by Kronecker and Stirling in their experiments on the heart. "A 

 glass T-tube is provided at the crossing point with a small opening (a). The vertical tube (b) 



Fig. 397. Fig. 398. 



Fig. 397. Plug key. Fig. 398. Capillary contact, e, vibrating platinum style adjustable 

 by/ and g and dipping into mercury at a ; b, bent tube filled with mercury, into which dips 

 a wire (d) ; a, opening in cross tube (c). 



is bent in the form of a U, and filled so full with mercury that the convex surface of the latter 

 projects within the lumen of the transverse tube (c). One end of c is connected with a Mariotte's 

 flask containing diluted alcohol, and the supply of the latter can be regulated by means of a 

 stop-cock. The fluid flows over the apex of the mercury and keeps it clean. The vibrating 

 platinum style (e) is attached to the end of a rod, which in turn is connected with the positive 



