40 EXPERIMENTAL GENERAL PHYSIOLOGY 



(2) To change the course of the current. Set up apparatus with 

 cross-bars removed as shown in Fig. 18. What course will the 

 current take when the bridge is turned toward a bf What course 

 when the bridge is turned toward c df 



(3) Ponl's commutator may be used as a simple mercury key 

 (Fig. 19). Is the current open or closed when the commutator 

 bridge is turned toward af How may the current be opened or 

 broken ? 



(d) Work Done by the Electric Cell. The experiments performed 

 show that the galvanic cell may, under proper conditions, liberate 

 energy. This energy is called electricity. But the immediate source 

 of the particular electric energy liberated in the foregoing experiments 

 is the latent chemical energy represented in the plates and liquids 

 of the cell. 



Under the conditions produced in the working galvanic cell the 

 latent chemical energy is transformed, and at the same time liberated 

 as electric energy. This liberated electric energy may make itself 

 manifest in the contact spark, in moving the galvanoscope needle, 

 or in lifting the armature of a magnet. In the last case mentioned 

 it would not be difficult to determine the amount of work done, 

 though it might be somewhat difficult to determine the amount of 

 work which a cell is capable of performing in a given time. If one 

 were to weigh the copper plate before and after using the cell, one 

 would find that it had increased in weight. This increase in weight 

 is an index of the amount of chemical action in the cell of the latent 

 chemical energy which has been transformed into electric energy. 



The amount of electrolysis must be, then, an index of the amount 

 of current that is afforded by a cell or battery. For example, if 

 the negative pole of a' cell be attached to a silver or platinum cup 

 containing pure nitrate of silver, and the positive pole be attached 

 to a piece of pure silver which is immersed in the silver nitrate solu- 

 tion, it will be found that one ampere of current will uniformly 

 deposit 0.001118 gm. of silver upon the cup in one second of time. 

 This brings us to the question of the units of electric measurements. 



(e) Electric Units. The electric energy available at any point 

 in a circuit i.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 



E. M. F.; E 



formula, C ' p ' - C = p-. It is impossible for the physicist to 

 zC -tC 



make any progress in the study of electric energy without arbitrarily 

 assuming units of measurement for current, for electromotive force, 

 and for resistance. 



(1) Current is measured in amperes. A current of 1 ampere deposits 

 upon the negative electrode of a galvanic cell or battery 1^0.001118 

 gm. of silver per second, or 4.025 gm. per hour. (See above.) 



