638 



PRINCIPLES OF GENERAL PHYSIOLOGY 



to make any impression on a heavier one. Just as a slight impulse would not 

 cause a visible movement of a cannon ball, but might give considerable motion 

 to a pith ball. This is especially to be taken account of when the electrical change 

 is subject to rapid alterations. The moment of inertia of the moving parts should, 

 therefore, be as small as possible. 



Damping. Suppose that a short-lasting electrical current has set one of 



these systems into movement. 

 It is clear that, unless there is 

 some influence to bring it to 

 rest, the movement will continue 

 after the current ceases. Now 

 our object is to obtain as true as 

 possible a record of the time 

 course of an electrical effect. 

 When a wire forming part of an 

 electrical circuit moves in a 

 magnetic field, a current is 

 developed in it in such a direc- 

 tion as to oppose further move- 

 ment. This will occur whether 

 the wire is conveying a current 

 already or not. In the 

 D'Arsonval and the string gal- 

 vanometers, therefore, as long as 

 the circuit is closed, any move- 

 ment of the wire produces in it 

 a current tending to stop its 

 movement. The extent of the 

 opposing current depends, by 

 Ohm's law, on the resistance of 

 the circuit, hence also the damp- 

 ing effect. In physiological 

 work this resistance is very 

 large, hence the damping is not 

 any greater than required. In 

 fact, in the D'Arsonval instru- 

 ments, it is usually necessary to 

 add a short circuit to increase 

 the damping; in the beautiful 

 galvanometer of Moll (1913), 

 made by Giltay of Delft, the 

 damping is regulated by altering 

 the strength of the current pro- 

 ducing the magnetic field. In 

 the Kelvin instrument, the 

 damping is usually effected by 

 air resistance to the movement 

 of a vane on the magnet system. 

 When the moving system is 

 heavy, the vane is some- 

 times made to move in a bath 

 of oil. 



Period of Vibration. Since the moving system must be brought back to 

 its resting position by some force, such as the magnetism of the earth or the 

 torsion of a wire, there will be oscillations similar to those of a pendulum. These 

 would prevent the real value of a deflection from being estimated, so that, 

 except for special purposes, the movement is made as nearly as possible 

 " aperiodic " by appropriate damping. When this is the case, the deflection 

 is reached without vibration 'around it. This is associated, however, with a 



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