NOTES ON SOME OF THE APPARATUS 471 



key the muscle is stimulated by a break-shock only. This is called 

 "cutting out" the shock not desired. 



The common error at first is having pressed down the simple key to 

 at once release it, forgetful that this gives two shocks instead of one. 

 The simple key must always be held down (closed) until the desired 

 reaction has completed itself; then the break-shock will do little harm save 

 toward wearying the muscle or other material. 



The rhythmic chronograph consists of a flat steel spring (vibrating 

 at any one of many rates per second and breaking the circuit at each 

 double vibration) actuating the electromagnetic signal. Used in one 

 way it breaks the signal-circuit once a second (caution as to the eyes). 

 Used the other way (horizontally) it actuates the signal either fifty times 

 per second, ten times per second as desired, or at any other rate near 

 these. 



The non-polarizable electrodes must be used especially in experiments 

 on nerves whenever delicate quantitative work is undertaken. These 

 prevent disturbance of the ions of the tissues by the chemical action be- 

 tween their salines and the metal of wires, etc. The porcelain boot- 

 electrode is most convenient when the moist-chamber is used, and the 

 camel's-hair brush form at other times. In either form no metal is 

 in contact with the tissue, the electricity being conducted by Ringer's 

 fluid in the boot or the brush. 



The capillary electrometer (A) measures very delicate electric currents 

 and (B) shows their direction. It is in general only in research-work 

 that we care to know the exact values of these electric currents, for they 

 are not known to be physiological ; but their direction is of more importance. 

 The movement of the end of the column of mercury in the capillary tube 

 is in the direction of the current, the surface-tension, usually holding it 

 still, being then lessened or else increased as the case may be. If the 

 current be down the tube, the tension is lessened; if up, it is increased. 

 A form with the capillary large enough to be seen readily without a lens 

 is often a convenience and fully precise enough for class-work purposes 

 in schools with the present too-short course of four years. 



Pohl's commutator or pole-changer is used in three ways : (A) To change 

 the direction of a current. The cross-wires then are in place, the battery- 

 wires (afferent) attached to the posts of the rocker, and the efferent 

 or stimulating wires attached to the pair of posts on one side or other of 

 the rocker. (B) To shift a battery-current into either of two efferent 

 currents at will, the cross-wires are removed and the battery-wires fastened 

 in the two pairs of posts (" rocker-posts") remaining. (C) As a simple 

 key, the cross-wires are removed and the wires of the circuit concerned 

 are placed one in a rocker-post and the other in a post adjacent to it. 



The rheocord is used to subdivide a weak current so that a whole or 

 any fraction of it may be employed; also for gradually increasing or 

 decreasing the strength of a weak current used as a stimulus. In order 

 to reduce the strength of a current, place the two wires carrying the 

 current from the dry-cell in the two corner binding-posts, and attach 



