76 CHEMICAL PHYSICS. 



Whenever the plates are connected by the conducting wire elec- 

 tricity passes from the anode through the wire to the cathode, and 

 through the liquid back to its point of origin. This continuous 

 motion is called the electric current, and the different parts through 

 which the current passes are known as the circuit. Whenever the 

 connection is broken at any point the circuit is said to be open ; 

 otherwise it is closed. If the circuit be open, both plates become 

 charged with positive and negative electricity, respectively ; but as 

 there is no conductor to carry off these charges, further accumulation 

 stops, and chemical action ceases until the circuit is again closed. 



Whenever two or more galvanic cells are connected so as to use in 

 one circuit the electricity generated by all the cells, the arrangement 

 is spoken of as a galvanic battery; at present this term is likewise 

 applied to a single cell. 



Electromotive force. The fact that electricity flows continuously 

 in a closed circuit containing a galvanic cell shows that the cell has 

 the power of setting electricity in motion, and this power is desig- 

 nated as electromotive force (E. M. F.), electrical tension, or potential. 

 It is the result of the tendency to re-establish equilibrium, and 

 depends upon the difference in the electrical condition of the two 

 plates ; the greater this difference the greater the E. M. F. 



The action of an electric battery may be compared to a pump sending water 

 from a reservoir through a pipe to a higher elevation ; the water will return to 

 its former level with a certain force, depending on the height to which it has 

 been lifted, and the water while descending can be made to do mechanical 

 work turn a wheel or set in motion other machinery. Similarly the cur- 

 rent of electricity on its return trip can be made to do work, and the quantity 

 of it depends on the E. M. F. generated by the battery. 



Electric units. For obvious reasons, it is desirable to measure the 

 intensity and quantity of electric currents similarly as heat or other 

 forms of energy are measured. The three chief units adopted for 

 such measurements are named, in honor of three great pioneers in 

 electricity, the ohm, the ampere, and the volt. 



It has been stated that we differentiate between conductors and 

 non-conductors, but even the best conductors offer resistance to the 

 passage of an electric current. The unit of this resistance is called 

 the ohm, and represents the resistance to a current in a column of 

 mercury having a section of 1 square millimeter and a length of 

 106.28 centimeters, at a temperature of C. For practical pur- 



