34-6 Dynamic Theory. 



net pole placed at the center. This absolute unit is too large, and in practice is difficult 

 to be realized. So the practical unit of current, which is called the ampere, is fixed at 

 one-tenth of the absolute unit ; and the measurement is made by means of the standard 

 galvanometer. This instrument is essentially a magnetic needle suspended in the mid- 

 dle of a circular coil of wire, the coil to be set upon edge so that its plane stands parallel 

 with the magnetic meridian. Its action upon the needle when a current is passed 

 through the coil, tends to cause the needle to take a position at right angles to the coil. 



The magnetism of the earth tends to keep the needle in the Earth's magnetic merid- 

 ian, while the current of electricity through the coil tends to deflect the needle from the 

 meridian. The force of the earth's magnetism varies in different localities, so that the 

 ampere-meter must be different in different localities to suit. The more intense the 

 magnetic current of the earth is, the closer must the current through the galvanometer 

 be to the needle, in order to deflect it to the same extent. Hence, where the earth-cur- 

 rent is weak the diameter of the coil in the instrument is made large. Thus, at London 

 the intensity of the earth current is .180, and the diameter of the coil is 6.87 inches. 

 At New Orleans the earth current is .280, and coil 4.42 inches. When the current 

 through this is just strong enough to deflect the needle 45 its force is that of one am- 

 pere. The tangent of 45 is the same as the radius. The natural tangents of other de- 

 grees of deflection in the ampere-meter, indicate amperes directly thus a deflection of 

 63.30=2 amperes, 71.35 =3 amperes, 76=4 amperes, 78.45' = 5, &c. "A current of 

 one ampere will cause the deposit in one hour 1.174 grams, or 18.116 grains, of copper in 

 a copper electrolytic cell. It will in one hour deposit 4.024 grams, or 62.52 grains, of 

 silver in a silver cell." (Thompson.) 



As stated above, every field of magnetic or electric force is such by virtue of lines of 

 force which are supposed to traverse it, as shown by the manner in which iron filings 

 are arranged by a magnet. For the sake of convenience it is assumed that each line 

 represents the force of one dyne, and that the intensity of the field depends on the num- 

 ber of these lines in a given area or section, say one centimeter. When the magnetic 

 pull on a unit magnet pole is, say 40 dynes, then, it is said there are 40 lines of force 

 within a cross section of a square centimeter. 



Electromotive force depends upon the number of magnetic lines of force that are "cut" 

 by a conductor moving in a field of magnetic force. (See fig. 136.) The dyne unit of 

 magnetic force is an obvious measure of Electromotive force, therefore, and would be 

 Adopted if it were not far too small for practical work. What has been adopted is a unit 

 composed of one hundred millions of the aforesaid absolute units. This great unit of 

 Electromagnetic force is called a Volt. A definition of one volt is, therefore, the electro- 

 motive force generated by a conductor cutting across a hundred million magnetic lines 

 per second. This is measured in practice by any cell of constant known force. A Dan- 

 iell's cell has a force of about 1 1-10 volts. A cell invented by Latimer Clark, is very con- 

 stant at about 1 4-10 volts. The Ohm, or unit electrical resistance, is made necessary by 

 the fact of resistance to the passage of a current in every conductor however good. 

 The actual available current for work is reduced by the resistance of the conductor, and 

 is the quotient resulting from dividing the electromotive force by the resistance. The 

 Ohm, or unit of .resistance, is fixed at one thousand millions of C, G, S units. 1 volt 

 divided by 1 ohm is equal to 1 ampere. An electrical Cfondenser is an apparatus for 

 holding electricity in a potential condition. A Leydeii jar is a condenser. The capacity 

 of a condenser is measured by an absolute unit called a Farad. A condenser which 

 holds one Coulomb at a potential of one Volt, has a capacity of one Farad. This is too 

 large a measure for practical use, and so the little farad, or Microfarad, is used. It takes 

 a million Microfarads' to make one Farad. 



The amount of work done in one second by an electrical machine is expressed by a unit 

 called a Watt, which is the same as one volt multiplied by one ampere. Calculation 

 shows that 746 watts equal one horse-power. If a certain arc lamp has a pressure or po- 

 tential of 57 volts between the lamp terminals, and the current is ten and a half am- 

 peres, the power expended is 598 and a half watts, or about eight-tenths of a horse- 

 power. Another " unit " is one of 1,000 watts, equal to one and a third horse-power, and 

 is used to designate the power of dynamos. A machine having the power of 8,000 watts 

 is an eight "unit " machine. 



