ELECTRICAL KEYS RHEOCHORD. 



163 



binding screw is similarly connected with the little metallic peg 

 at the right-hand end of the fig. 



7. Plug-Key (fig. 89). Two brass bars are fixed to a piece of vulcanite. 

 The circuit is made or broken by inserting a brass plug between the bars. 

 Each brass bar is provided with two binding screws, to which one or two 

 wires may be attached, so that it can be used like a Du Bois key, either by 

 the first or second method. 



8. The " Trigger or Turn-Over Key" is referred to in Lesson XXXV. 



9. For Brodie's " Rotating Key," see Lesson XXVIII. 



Means of Graduating a Galvanic Current. Besides altering 

 the number, arrangement, or size of the cells themselves, we can 

 use a simple rheochord to divide the current itself, the battery 

 remaining constant, so that weak constant currents of varying 

 strength can thus be easily obtained. 



10. The Simple Rheochord consists of a brass or German-silver 

 wire, about 20 ohms resistance and i metre in length, stretched 

 longitudinally along a board, and with its ends connected to 

 binding screws and 

 insulated (fig. 90). On 

 the wire there is a 

 " slider " which can 

 be pushed along as 

 desired. Apparatus. 

 Simple rheochord, 

 Darnell's cell, detector, 

 Du Bois key, five 

 wires. 



(a.) Arrange the ex- 

 periment as in fig. 90. 

 When the slider S is 

 hard up to W, practically all the electricity passes along the wire 

 (W, K) back to the battery. 



(6.) Pull the slider away from W, and in doing so, the resist- 

 ance in the detector circuit is diminished, and some of the elec- 

 tricity passes along the detector circuit or the "deriving circuit" 

 and deflects the needle. The deflection is greater but not pro- 

 portionally so the further the slider is removed from W. The 

 deflection is nearly proportional to the distance of the slider from 

 W, when the resistance in the detector circuit is great compared 

 with that of the rheochord, which is, of course, the case when a 

 tissue occupies the place of the detector. 



FIG. 90. Scheme of Simple Rheochord. B. Battery ; 

 K. Key ; W, R. Wire ; S. Slider ; D. Detector. 



