300 Steady Currents in Linear Conductors [OH. ix 



Units. 



345. On the electrostatic system, a unit current has been defined to be 

 a current such that an electrostatic unit of electricity crosses any selected 

 cross-section of a conductor in unit time. For practical purposes, a different 

 unit, known as the ampere, is in use. The ampere is equal very approxi- 

 mately to 3 x 10 9 electrostatic units of current. 



To form some idea of the actual magnitude of this unit, it may be stated that the 

 amount of current required to ring an electric bell is about half an ampere. About 

 the same amount is required to light a 16 c. p. 100- volt incandescent lamp. 



As an electromotive force is of the same physical nature as a difference 

 of potential, the electrostatic unit of electromotive force is taken to be the 

 same as that of potential. The practical unit is about -^ of the electrostatic 

 unit, and is known as the volt. 



It may be mentioned that the electromotive force of a single voltaic cell is generally 

 intermediate between one and two volts ; the electromotive force which produces a 

 perceptible shock in the human body is about 30 volts, while an electromotive force 

 of 500 volts or more is dangerous to life. Both of these latter quantities, however, vary 

 enormously with the condition of the body, and particularly with the state of dryness 

 or moisture of the skin. The electromotive force used to work an electric bell is 

 commonly 6 or 8 volts, while an electric light installation will generally have a voltage 

 of about 100 or 200 volts. 



The unit of resistance, in all systems of units, is taken to be a resistance 

 such that unit difference of potential between its extremities produces unit 

 current through the conductor. We then have, by Ohm's Law, 



, difference of potential at extremities 



current = - - (268). 



resistance 



In the practical system of units, the unit of resistance is called the Ohm. 

 From what has already been said, it follows that when two points having a 

 potential-difference of one volt are connected by a resistance of one ohm, the 

 current flowing through this resistance will be one ampere. In this case the 

 difference of potential is ^^ electrostatic units, and the current is 3 x 10 9 

 electrostatic units, so that by relation (268), it follows that one ohm must be 



equal to - - electrostatic units of resistance, 

 y x j.u 



Some idea of the amount of this unit may be gathered from the statement that 

 the resistance of a mile of ordinary telegraph wire is about 10 ohms. The resistance 

 of a good telegraph insulator may be billions of ohms. 



KIRCHHOFF'S LAWS. 



346. Problems occur in which the flow of electricity is not through 

 a single continuous series of conductors : there may be junctions of three or 

 more conductors at which the current of electricity is free to distribute itself 



