Electrical Devices and How They Work 



VI. — Electrical metering instruments 

 By Peter J. M. Clute, B. E. 



ELECTRICITY is manifested to us 

 only through certain effects which 

 it produces. These effects may be 

 mostly classified under chemical, thermal, 

 magnetic, and static effects. 



Chemical effect is manifested by the 

 decomposition of a solution when a cur- 

 rent of electricity passes through it. 



Thermal effect is produced by the pas- 



-Collected goses 



Covering to prevent 

 evoporation; 



A gas voltameter consisting of two platinum 

 electrodes immersed in acidulated water 



sage of an electric current through a con- 

 ductor of appreciable resistance. 



Magnetic effect results from an electric 

 current in a conductor when brought into 

 a magnetic field, the field being produced 

 by a magnetic or other electric currents. 



Static effect is the attraction or re- 

 pulsion existing between highly electrified 

 bodies. 



These effects all occur in perfect ac- 

 cordance with definitely fixed natural 

 laws, and as a consequence they have been 

 utilized in the design of various instru- 

 ments for the measurement of electrical 

 qualities. 



Electrolytic meters, depending on the 

 chemical effect of the electric current, do 

 not indicate directly in amperes the cur- 

 rent flowing, but are used to determine 

 the quantity of current which passes in a 

 definite period of time. Thus, they should 



be termed coulomb, or ampere-hour, 

 meters. In Fig. 1 is shown the gas volta- 

 meter or electrolytic meter. The gas 

 voltameter consists of two platinum 

 electrodes immersed in acidulated water, 

 so arranged that all the evolved gas would 

 be collected in a graduated cylinder. It 

 can thus be demonstrated that the 

 amount of gas is entirely independent of 

 any consideration by the quantity of cur- 

 rent flowing. By using solutions of 

 different metals for electrolj'te, it can be 

 shown that the weight of metal deposited 

 on the positive electrode is always pro- 

 portional to the amount of current in the 

 circuit. 



The commercial electrolytic instrument 

 depends on the principle that current 

 passing through a volume of water de- 

 composes it into its constituent gases, 

 hydrogen and oxygen. This decomposi- 

 tion is directly in proportion to the cur- 

 rent passing, one ampere-hour decompos- 

 ing .338 gms. of water. The current 

 flows through a volume of water contained 

 in a properly graduated tube, the change 

 in volume of water in a given time, indi- 

 cated by the difference in levels, showing 

 the amount of current which has passed. 



The commercial use of electrolytic 

 meters is quite limited, and in most cases 

 they have been superseded by direct- 

 reading electro-magnetic type meters. 



The thermal effect of electric current is 

 utilized in some instruments to- measure 

 the current passing, by means of the ex- 

 pansion of a wire. The heating of a wire 

 is proportional to the square of the cur- 

 rent and the resistance of the wire, when a 

 current is passed through it. 



Hot-wire instruments are used to meas- 

 ure current or differences in potential by 

 the heating effect of the current. Refer- 

 ring to Fig. 2, a long wire, of high re- 

 sistance, non-oxidizable metal, has one 

 end fastened to a plate, passes over a 

 pulley and back to the plate. A spring 

 attached to the plate keeps the wire 

 stretched taut. When a current is passed 

 through the wire, it heats and expands, 



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