226 ELECTRO-MOTIVE ENGINES 



of power tears a decreasing ratio to the power employed, and at last, the addition of 

 five elements was not found to produce an increase of effect equivalent to the value of 

 one element. In all experiments, therefore, on electro-magnetic machines, the experi- 

 mentalist has first to determine the utmost power which the soft iron is capable of 

 assuming, in relation to 1st, the number of coils of wire on the iron ; and 2nd, tho 

 number of elements employed in the exciting source the voltaic battery. Tho li-n^th 

 of the iron and its thickness are also points demanding special considerations from the 

 constructor of an electro-magnetic machine. 



There remains now to examine the production of the power, Electro-Magnetism. 



The electro-mechanician is dependent upon his battery, in the same way as a steam 

 engineer is dependent upon his fire and his boiler, for tho production of mechanical 

 effect. 



Voltaic batteries vary in their effects, and hence arise statements which differ widely 

 from each other, as to the result obtained, by the destruction (? change of form) of a 

 given quantity of metal in the battery. 



Dr. Botto states, that 45 Ibs. of zinc, consumed in a Grove's battery, are sufficient to 

 work a one-horse power electro-magnetic engine for twenty-four hours. 



Mr. Joule says the same results would have been obtained, had a Daniell's battery 

 been used, by the consumption of 75 Ibs. of zinc. 



It is impossible, on the present occasion, to enter into tho theory of the voltaic 

 battery, or to describe the varieties of arrangement which have been adopted for 

 generating (developing) electrical force in the form of a current, with the greatest 

 effect, at the smallest cost. 



On this point the evidence of Jacobi maybe quoted : ' With regard to tho magnetic 

 machine, it will be of great importance to weaken the effects of tho counter current, 

 without at the same time weakening tho magnetism of the bars. It is the alternate 

 combination of the pairs of plates in tho voltaic pile, which permits us to increase the 

 speed of rotation at will. We know the magnetic power of tho current is not sensibly 

 augmented by increasing the number of the pairs of plates, but the counter current is 

 considerably weakened by its being forced to pass through a great many layers of 

 liquid. In fact, on using twelve voltaic pairs, each, half a square foot, instead of four 

 copper troughs, each with a surface two square feet, which I had hitherto used, the 

 speed of rotation rose at least 250 or 300 revolutions in a minute.' 



Mechanical force, whether obtained in the form of man-power, horse-power, steam- 

 power, or electrical power, is the result of a change of form in matter. In the animal, 

 it is the result of muscular and nervous energy, which is maintained by the due supply 

 of food to the stomach. In the steam-engine, it is the result of vapour-pressure, which 

 is kept up by the constant addition of fuel to the fires, under the boilers. In tho 

 magnetic machine, it is the result of currents circulating through wires, and these 

 currents are directly dependent upon tho chemical change of zinc or of some other 

 metal in the battery. Then, 



Animal-power depends on food. 

 Steam-power depends on coal. 

 Electrical power depends on zinc. 



An equivalent of coal is consumed in the furnace that is, it unites its carbon with 

 oxygen to form carbonic acid, and its hydrogen with oxygon to form water, and during 

 this change of state tho quantity of heat developed has a constant relation to tho 

 chemical action going on. 



Mr. Joule has proved by a series of most satisfactory experiments, that : ' Tho 

 quantity of heat capable of increasing tho temperature of a pound of water by one 

 degree of Fahrenheit's scale is equal to, and may bo converted into, a mechanical forco 

 capable of raising 838 Ibs. to the perpendicular height of one foot.' 



Mr. J. Scott Russell has shown that in the Cornish boilers, at Huel Towan and the 

 United Mines, the combustion of one pound of Welsh coal evaporates of water, from 

 its initial temperature, 10'58 Ibs. and 10'48 Ibs. respectively. 'But,' says Mr. Joule, ' wo 

 have shown that one degree is equal to 838 Ibs. raised to tho height of one foot. 

 Therefore the heat evolved by tho combustion of one pound of coal is equivalent to tho 

 mechanical force capable of raising 9,584,206 Ibs. to tho height of one foot, or to about 

 ten times the duty of the best Cornish engines.^ 



Such are the conditions under which heat is employed as a motive power. An 

 equivalent of zinc is acted on by the acid in the colls of the battery, and is oxidise. 1 

 thereby. In this process of oxidation a given quantity of electricity is set in motion ; 

 but the quantity available for use falls very far below the whole amount developed 

 by tho oxidation of the zinc. The electricity, or electrical disturbance, is generated 

 on the surface of the zinc ; it passes through tho acidulated fluid to tho copper plate 

 or platinum plate, and in thus passing from one medium to another it has to overcome 



