354 GENERAL DIRECTIONS. 



bound, and therefore with great accuracy, in any position 

 whatever. 



VII. Commutator. This is useful for changing the direc- 

 tion of a current when the effects of constant currents are 

 being studied. Any form of commutator ma} r be used, pro- 

 vided that the current can easily be cut off altogether, as well 

 as reversed in direction. A very convenient form is that 

 represented in fig. 297, -in which, when the handle is horizontal, 

 the current is cut off from the electrodes altogether; and a 

 different direction given to the current according as the handle 

 is raised or lowered. The wires from the battery should be 

 brought to the upper, and those from the electrodes to the 

 lower binding screws. 



VIII. Rheochord. This instrument is directed to be used 

 in the following pages simply for the purpose of causing defi- 

 nite changes in the amount of a constant current under use. 

 Fig. 298 represents a convenient form of the rheochord of Du 

 Bois Raymond. 



Bring the wires from the battery to the binding screws at 

 the top of the board and those from the electrodes to the same 

 scivws. If all the plugs are in place and the travelling mer- 

 cury cups close up to the top of the board in direct contact 

 with the brass, the resistance to the current from the battery 

 offered by the rheochord compared with that offered by the 

 circuit of the electrodes is practically nil, and consequently all 

 the current passes through the former and none through the 

 latter. If the mercury cups be moved on their platinum wires 

 a little distance down the board, there will be no passage for 

 the current from one side of the rheochord to the other, ex- 

 cept through such a length of the two platinum wires as lies 

 between the cups and the brass plate. But these thin wires 

 offer :i certain resistance to the passage of the current, and 

 consequently a proportionate fraction of the total current from 

 the battery is thrown into the circuit of the electrodes. By 

 sliding the mercury cups various distances down the graduated 

 board, small differences of resistance in the rheochord are es- 

 tablished, and consequentl}'- slightly differing fractions of the 

 total current thrown into the circuit of the electrodes. If one 

 of the plugs be removed, a certain amount of resistance is 

 suddenly introduced into the rheocord, and consequently a 

 certain amount of the current is suddenly thrown into the 

 circuit of the electrodes. With the different plugs different 

 amounts of resistance are introduced into the rheochord, and 

 different amounts of the current thrown into the circuit of the 

 electrodes. The several plugs are all numbered as multiples 

 of the resistance offered by the total length of the platinum 

 wires on which the cups travel. Thus if the resistance offered 

 by the rheochord when the cups are quite at the bottom of the 



