METHODS OF ELECTRICAL STIMULATION 35 



trometer is necessary. In the galvanometer, the 

 points of different potential are connected by a 

 coil of wire near which is suspended a magnet. 

 When the circuit is completed, the electrical 

 energy acts on the suspended magnet by induc- 

 tion, and deflects it to an extent proportionate 

 to the difference of potential. In the capillary 

 electrometer, which is the electrometer preferred 

 here, a capillary tube filled with mercury and 

 sulphuric acid dips in a wider tube which con- 

 tains sulphuric acid. The points the potential 

 of which is to be measured are connected with 

 the mercury and the acid respectively. When 

 the connection is made, the tension of tHe sur- 

 face of mercury in contact with the acid changes, 

 causing the mercury to move in the capillary. 

 The change in surface tension is proportional to 



diminishes, just as water can be forced more easily through 

 wide channels than through narrow ones. The unit of electri- 

 cal resistance is the ohm. The precise definition of these units 

 is as follows : N 



A volt is the electromotive force that, steadily applied to a 

 conductor whose resistance is one international ohm, will pro- 

 duce a current of one international ampere. The practical 

 ampere (coulomb) is the unvarying current which, when passed 

 through a solution of nitrate of silver in water, deposits silver 

 on the cathode, or negative pole, at the rate of 0.001118 gram 

 per second. The ohm is the resistance offered to an unvarying 

 electrical current by a column of mercury at the temperature 

 of melting ice, 14.4521 grams in mass, of a constant cross- 

 sectional area, and of the length of 106.3 centimetres. 



