GENERAL PHYSIOLOGY. 31 



Cup containing pure nitrate of silver, and the posi- 

 tive pole be attached to a piece of pure silver which 

 is immersed in the silver nitrate solution, it will be 

 found that one ampere of current will uniformly de- 

 posit 0.001118 gm. of silver upon the cup in one 

 second of time. This brings us to the question of 

 the units of electrical measurements. 

 e. Electrical units. The electrical energy available at 

 any point in a circuit, 2. e. , the current, as it is called, 

 is, according to Ohm's law, equal to the liberated 

 energy the electromotive force divided by the 

 total resistance of the circuit. This is expressed in 

 Ohm's formula, C = ^^ C = | It is im- 

 possible for the physicist to make any progress in 

 the study of electrical energy without arbitrarily 

 assuming units of measurement for current, for 

 electromotive force and for resistance. 

 ^1) Current is measured in amperes. A current of 

 one ampere deposits upon the negative electrode 

 of a galvanic cell or battery 0.001118 gm. of silver 

 per second, or 4.025 gm. per hour. [See above.] 

 A concrete idea of the ampere may be gained 

 from the fact that the small sized Daniell cell 

 produces a current of about ^ ampere when the 

 external resistance is reduced to a minimum. 

 (3) Resistance is measured in ohms. An ohm is 

 that amount of resistance, opposed to the trans- 

 mission of electrical energy, by a column of mer- 

 cury 1 sq. mm. in cross section and 106.3 cm. 

 in length. For general purposes an ohm re- 

 sistance is that of a pure silver wire 1 mm. 

 in diameter and 1 meter in length. 

 (3) Electromotive force is measured in volts. 



A volt is that amount of electrical energy which 



