BY DR. MICHAEL FOSTER. 355 



board, but all the plugs in place, be taken as the unit, the re- 

 moval of the plug marked 5 will suddenly introduce in addi- 

 tion five times that amount of resistance, and so send a pro- 

 portionate amount of the current through the circuit of the 

 electrodes. 



IX. The Double Key or Wippe. This is very con- 

 venient when it is desired to throw a given current from one 

 pair of electrodes into another. It is represented in fig. 299, 

 in which it is seen that the mercury cups belonging to the 

 binding screws 1 and 2 are connected by a handle of which the 

 central part is of insulating material, the ends of thick copper 

 wire. Each of the copper wires is crossed just as it enters 

 the handle by an arch of the same material ; one end of each 

 arch dips into one of the mercury cups 3 and 4, when the 

 handle is thrown to the right as in the figure. The wires from 

 one pair of electrodes are to be brought to the binding screws 

 3 and 4, those from the other to the screws 5 and 6. The small 

 cups on the surface are to be filled with mercury, and the wires 

 from the battery or induction coil, etc., brought to the screws 

 1 and 2, and the straight cross wires between 3 and 6 and 4 

 and 5 taken away. By throwing the handle to the right, the 

 current from the battery is sent into the wires connected with 

 the screws 3 and 4 ; by throwing the handle to the left, into 

 the wire connected with the screws 5 and 6. 



X. The Marking Lever. This is a two-armed, flat, 

 metal lever, fig. 2Y5 a a', working vertically on the pivot 6, the 

 arm a being considerably heavier than a f . The pivot is elec- 

 trically continuous with the small brass pillar c, where binding 

 screws receive a wire or wires from & battery, induction coil, 

 electrode, etc. The pillar c is placed on one side of the sup- 

 port d, made of non-conducting material, which by e can be 

 clamped to any stand, either vertically or horizontally. On 

 the other side of the support is a similar pillar/" (also bearing 

 binding-screws), on which the arm a of the lever rests ; g is a 

 weak spring, which serves to catch the end of the lever when 

 thrown up ; h is a slip of gutta-percha or India-rubber at the 

 end of the lever bearing the marking needle or pen. 



If c be connected with one of the poles of the batteiy, and 

 f with one end of the primary coil, when the lever is down and 

 horizontal, the arm a being in close contact with the pillar, the 

 current passes along the lever from c to /. When, therefore, 

 the arm a of the lever is suddenly tilted up, which can easily 

 be done with the point of the finger, the current is suddenly 

 broken ; and the moment of the breaking is indicated on the 

 registering surface by the descent of the marking point of the 

 lever. When it is desired to mark the making rather than the 

 breaking of a current, the two positive (or negative) wires 

 must be brought to the binding screws of/, and the two nega- 



