45, COENHILL, E.G., AND 122, EEGENT STREET, W., LONDON. 565 



OHM'S LAW. 



Ohm assumed the passage of the electric fluid from one section to another of the 

 connecting wire to be due solely to the difference of electric tension between the two 

 sections ; he further assumed the quantity of electricity transmitted to be proportional to 

 this difference of tension, and from these fundamental assumptions he deduced the laws of 

 the voltaic circuit. These laws may be briefly stated thus : 



a. The strength of the current is directly proportional to the electromotive force. 



I. The strength of the current is inversely proportional to the resistance. 



c. If the wire which unites the two poles of battery be of the same material, and of the 



same thickness throughout, the " electric fall " is the same throughout the wire. 



d. If the wire be of the same material but of different thicknesses, the " fall " is steeper 



on the thin wire than on the thick. The ' fall " is inversely proportional to the 

 cross section of the wire. 



e. If the poles be connected by two wires of the same thickness but of different resisting 



powers, the "electric fall " is steeper on the more resisting wire. The "fall" is 

 directly proportional to the specific resistances of the wires.* 



It has been found that when the same current is passed respectively through a short and 

 through a long wire of the same material, its action on the magnetic needle is less in the 

 latter case than in the former. Ohm accordingly supposed that in the latter case there was 

 a greater resistance to the passage of the current than in the former ; and he proved that 

 the resistance is inversely proportional to the intensity of the current. 



On these principles Ohm founded the celebrated law which bears his name, that 

 The intensity of the current is equal to the electromotive force divided by the resistance. 



"Ci 



Which is expressed by the simple formula I= R 

 Where I is the intensity of the current, E the electromotive force, and R the resistance.! 



ELECTRIC UNITS. 

 The Unit of Resistance is 



1 Ohm == 1 British Association Unit = 1 B. A. U. = 10 Million Units of resistance, or : 

 1 Siemen's Unit = 1 S.';E., the resistance of a prism of mercury, 1 metre long, and 1 



square millimetre in section at C. 

 1 Ohm = 1-070 S.E. 

 1 S.E. = -935 Ohm. 

 1 Megohms = 1 million Ohms. 



The Unit of Tension is == 1 Volt = 100,000 Units of Tension. 

 The Unit of Quantity = 1 Weber. 

 The Unit of Charge is = 1 Microfarad = 1 Millionth part of a Farad.f 



t Electric Testing of Telegravh Cables. By Capt. V. Hoskisev-, Eoyal Danish Engineers. 

 1 Ohm = about one mile of Copper Wire, No. 16 B. W. Gauge; or 1 mile of No. 8 Iron Wire, B. W. 



Gauge a rough definition by a Workman. 



The laws of the voltaic circuit as enunciated by Ohm have been verified by Kohlrausch 

 and others, and found to be in strict accordance with his theory. 



THERMOMETER SCALES. 



The Zero or Freezing Point of the Centigrade and of Reaumur's Scale correspond 

 to 32o of Fahrenheit's Thermometer. 



The Boiling Point of the Centigrade Scale is 100 of Reaumur's Scale, 80 being 

 equivalent to 212 of Fahrenheit. 



EASY RULES FOE THE CONVEKSION OF SCALES. 



To convert Fahrenheit degrees into Centigrade, substract 32, multiply by 5, and divide 

 by 9. 



* See Tyndall's Notes on Electricity. t See Ganot's Elementary Treatise on Physics. 



