952 Transactions of the American Institute. 



15. The Units of Electric Measurement, 

 herein adopted, are those of the British Association, viz. : " ohm, 

 volt and veber." The "ohm" is the unit of resistance, and is equal 

 to the resistance of a prism of pure mercury, one square millimeter 

 section and 1.0486 meters long at 0° C. 



16. The "volt" is the unit of electro-motive force, which varies but 

 little from the electro-motive force of a standard Daniell's cell. 



17. The " veber " is the unit of strength, or quantity, or electro- 

 chemical equivalence of a current, as it is variously called, and repre- 

 sents that quantity of electricity which flows through a circuit having 

 an electro-motive force of one volt and a resistance of one ohm in one 

 second. 



18. One veber of electricity decomposes 



.00142 grains water, or develops 



.000158 grains hydrogen, or 



.1821 C.C. mixed gas, at a temp, of 



0° C, and barometric pressr. of 760 m / m .] 



e=l volt ^ , . 



— - — -. — =s=l veber per second. 

 r=l ohm 



19. Resistance of Conductors. 



To determine the resistance of any conductor, attach its extremities 

 to the screw-cups III and IV of the rheostat, one pole of the battery 

 to B of the galvanometer, and the other to I of the rheostat. 



The wire leading from II of the rheostat is connected with one of 

 the screw-cups one, two, three, four, of the galvanometer, as may be 

 required. 



For resistance of 1,000 ohms or more, the No. 1 screw-cup, with a 

 compound or intensity battery, is most suitable ; No. 2, from twenty 

 to 1,000 ohms ; No. 3, from two to twenty ohms ; and No. 4, with a 

 single large cell of battery (or, what is better, two or more cells 

 arranged for quantity) for very small resistances, two ohms or less. 



20. Before measuring small resistances, it is necessary to balance 

 the apparatus, as we would a scale-beam, before weighing small quan- 

 tities. To do this, connect III and IV by a short wire, such as may 

 be used in connection with the thing to be measured ; the rheostat 

 being fully and carefully plugged. If, now, on turning the switch to 

 the right, the needle goes up, it shows that the short wire does not 

 have resistance enough to balance ; therefore, a wire of larger resist- 

 ance must be selected ; on the contrary, if the needle falls back, plugs 



