522 PROF. JACOBI ON THE APPLICATION OF ELECTRO-MAGNETISM 



were always interrupted by some accident, and I think that the action 

 would otherwise have lasted still longer. The disengagement of gas 

 was very inconsiderable, and took place only at the surface of the nega- 

 tive plate. The velocity was always at first from 120 to 122 revolutions 

 in a minute, and decreased about half an hour after to 62 revolutions ; 

 a circumstance which is attributable to the commutator, which had not 

 then the present construction. During the rest of the time the motion 

 of the apparatus was remarkably uniform, making from 58 to 62 revo- 

 lutions in a minute. I must however confess that I have obtained so 

 extraordinary an effect only three times. There were always external 

 circumstances, dependent upon the form of the voltaic apparatus, which 

 counteracted the effect. I might be able to master most of these cir- 

 cumstances by constructing a new apparatus, the manipulation of which 

 M'ill be more convenient and the effect more certain. 



14. 



JE 



We have expressed by ^ = -^ the magnetic force of each section of 



a wire traversed by an electric current. This force is measured by 

 the deviation of the needle or by the magnetizing power of the con- 

 necting wire. By adopting 'the law of Faraday we may equally mea- 

 sure this current by the disengagement of the gas, which represents at 

 the same time the cost of maintaining in action a voltaic apparatus. If 



D be the quantity disengaged, we shall have D ^ ^j-. From this it 



It 



follows that recurring to the formulaj of article 8, the ceconomical 



effect may be expressed by the magnetic power of the whole extent of 



the connecting wire, divided by the development of the gas. This effect 



is in no respect changed either by the enlargement of the surface, or by 



the employment of various branches wound spirally around different bars 



of the same dimension. But by multiplying the helices, and uniting 



them to form a continuous wire, the ceconomical effect may be increased 



as much as we please. For the disengagement of the gas, in employ- 



E 



ing n helices or n units of length, will be expressed by Z) = r-; 



but we may put in action the magnetizing power of the whole ex- 

 tent of the conducting wire, and we shall have for the total force 



F = ; or —-:=«. When the magnetic bars ai'e intended to 



nr + r' D ^ 



produce a mechanical motion, the increase of the ceconomical effect 



will reach its limit ; since by multiplying the number of the bars, the 



weight of the apparatus and the friction of the pivots in the sockets 



will be at the same time increased, so that tliat effect can only be ex- 



jt* \ 71,7* lit 7* \71. f 



pressed by-=- z=n— r:< • The maximum of the ceconomical 



