ELECTRICITY, COMMON. 



ELECTRICITY, COMMON. 



700 



nomena of electricity being included in one or other of the grea 

 divisions ELECTRO-STATICS and ELECTRO-DYNAMICS. In the first o 

 these the electric fluid is regarded in a state of equilibrium or repose 

 and the phenomena of stationary charges and the laws of excitatio 

 and discharge are also studied ; while in the second division, which i 

 much the more extensive of the two, the laws of continuous discharge 

 or of electric currents, are chiefly considered. 



Although the electric force is greatly modified by the mode in \vhic 

 it is excited, yet the phenomena are so far related as to suggest th 

 idea of a common principle. Frictional electricity is eminently distin 

 guished by its tension, or power of overcoming resistance, whic 

 enables it to strike through air from one conducting surface tc 

 another, and to discharge itself with great facility from points, whereaf 

 the electricity of a powerful voltaic battery, owing to its low tension 

 has no striking distance either through air or in an exhausted receiver 

 Frictional electricity passes readily through rarefied and heated air 

 and through flame. In like manner, the voltaic discharge will tak 

 place through heated air, as when the charcoal points of the poles 

 of a powerful battery are gradually separated after contact. Wit] 

 a very powerful apparatus the discharge has been thus made to 

 pass through 4 inches of air, and in an exhausted receiver through 

 6 or 7 inches. Common and voltaic electricity have remarkable 

 heating powers, and in this respect magneto-electricity is not deficient 

 but neither thenno- nor animal electricity has as yet exhibitet 

 heating power. The magnetic powers of all the electricities are 

 strongly marked, especially those of voltaic electricity : all in like 

 manner deflect the magnetic needle, and all have the property o 

 imparting magnetism to steel. Common and voltaic electricity, espe 

 dally the latter, effect numerous chemical decompositions. Water 

 has been decomposed by the electricity of the torpedo, and also by 

 magnetic action, as well as by thermo-electricity. The shock producec 

 by common and by voltaic electricity becomes painfully exalted when 

 imparted by magneto-electricity. By means of thermo-electricity the 

 limbs of a frog have been convulsed ; and the severe shocks given by 

 the torpedo and electrical eel are well known. The spark common to 

 the frictional, voltaic, and magnetic fluids has been obtained from the 

 torpedo. From these and other points of comparison the identity ol 

 the five kinds of electricity seems to be made out, the differences ol 

 intensity and quantity being sufficient to account for then- supposec 

 distinctive qualities. Faraday has further strengthened the belief in 

 thuir identity by showing that the magnetic force and the chemical 

 action of electricity are in direct proportion to the absolute quantity of 

 the fluid of whatever intensity which passes through the galvanometer. 

 Thus the electric phenomena, connected as they are by well ascertained 

 laws, form the most complete and important addition to the physical 

 sciences that has been made since the time of Newton. 



The most important treatise that has been published of late years 

 on all the electrical forces is that by De la Rive, in three large volumes, 

 a translation of which has been published by C. V. Walker. This 

 work contains copious lists of authoritative memoirs ; but it should 

 be stated, that the views adopted differ in some important particulars 

 from those of English electricians. 



ELECTRICITY, COMMON, FRICTIONAL or FRANKLINIC. 

 The simplest and most usual mode of producing electricity is by 

 friction. When any two substances are rubbed against each other 

 briskly it is always produced ; but it is only a particular description of 

 bodies called non-conductors that retain it after it is thus produced so 

 as to exhibit its primary effects of attraction and repulsion. 



The production of electricity may be observed in a very familiar 

 manner thus : Tear up a piece of paper into small fragments, and place 

 them on a table ; then take a stick of sealing-wax, and rub it briskly 

 with a piece of flannel, or against the sleeve of a cloth coat, and imme- 

 diately after hold it near the fragments of paper ; these small pieces 

 will be soon observed to be agitated and the smaller to fly with 

 considerable velocity to the wax, to which having adhered for some 

 time, some will suddenly jump off, others which have touched the 

 stick edgeways will dangle from it for a considerable time, and 

 then fall off by their own gravitation when the electrical force has 

 sufficiently subsided. It was by observing that amber produced 

 similar effects after friction to those we have described that electricity 

 obtained its name (1j\(Krpov, amber). 



Glass is now more commonly employed to produce electricity 

 than wax or gum; and there is a striking difference in the kind of 

 electricity then generated, which we shall afterwards notice more at 

 length. These bodies are non-conductors, as they manifestly retain 

 their electricity beyond the moment of its production, and they are 

 inJalurt or intulatori, because a conducting substance will also retain 

 its electricity when communication with the earth, or with other con- 

 ducting substances, is cut off by means of non-conducting supports or 

 envelopes. 



Water is a conductor of electricity ; for if we immerse a conducting 

 insulated and electrised substance in water, it will be completely 

 deprived of its electrical properties. Perfectly dry airs or gases, on 

 the<other hand, are non-conductors ; for if not, none of the phenomena 

 iii'-iiUnned could have been observed, as the experiments are not made 

 in vacuo. We are not aware that it has yet been established whether 

 the vapoun of all substances are conductors (or all the gases non- 

 conductors). Aqueous vapour certainly is a conductor ; and therefore, 



when the air is impregnated with moisture, it is difficult to perform 

 any electrical experiments which require duration. [ELECTRIC.] 



Hence a conductor constitutes a channel by which the electricity 

 produced on a non-conductor will escape, and a non-conductor con- 

 stitutes an insulator by which the electricity communicated to a 

 conductor may be retained. Thus, if we place an electrified glass 

 tube on a tin stand, the metals being conductors, the electricity will 

 be gradually dissipated ; on the contrary, if we place an electrised tin 

 cylinder on a glass support in dry air, the electricity will be retained 

 for a long time. 



Nevertheless it is far from improbable that this is a question only of 

 degree ; that all substances are conductors ; and that the usual terms 

 conductor and non-conductor strictly mean quick conductor and slow 

 conductor. 



When an electrised body is brought near the skin where the power 

 of touch is delicate, a sensation is produced which has been compared 

 to that produced by the touch of a cobweb ; but instruments founded 

 on the laws of electric action have been constructed, which indicate 

 the presence of electricity in its most feeble state, and measure its 

 tension. [ELECTROMETER.] 



Previous to our study of the properties of electricity, on which the 

 construction of the best machines for procuring it in large quantities 

 depends, it will be useful to describe a simple apparatus, and one 

 easily constructed or procured, by which we may learn the first laws 

 of electrical action. 



A represents a glass cylinder of which the axis is supported on a 

 frame, and which is pressed against by a cushion stuffed with horse- 

 liair, and covered by an amalgam of zinc,. tin, and mercury ground up 

 with lard and spread over its surface; this cushion is mounted on 

 glass rod c, but attached to the cushion is a metal chain com- 

 municating with the table or the ground. At one extremity of the 

 axis of the cylinder is attached a handle by which it may be rapidly 

 turned round, and the friction which is generated against the cushion 

 will produce electricity on the surface of the glass, to guard which 

 against the action of the moisture of the air, the upper side of tho 

 cylinder may be lapped over by a piece of glazed taffeta in the direction 

 of the rotary motion of the cylinder. B is a long narrow and hollow 

 metallic cylinder standing on a glass support, and having at the extre- 

 mity near A a small cross bar garnished with points or teeth presented 

 rowards the cylinder. B is called the prime conductor in this apparatus, 

 >r cylinder electrical machine as it is called. When the cylinder A is 

 turned briskly round, the motion will be accompanied by a crackling 

 noise, and in the dark, streams of light of a yellowish or bluish hue 

 will be perceived directed to the several points on the projecting bar 

 of the conductor. B may be thus charged with electricity, and when 

 removed from the presence of the cylinder (taking it away by its 

 usulatiug support), it will retain its electrical properties (the longer 

 as the air is more free from moisture), and will by simple contact 

 lommuuicate a portion of its electricity to another isolated conducting 

 ubstance, or be discharged by touching one not isolated : if, with a 

 eeble charge, it be touched by the finger, a sensation like the pricking 

 f a needle will be felt, accompanied by a faint spark apparently 

 >enetrating the fingers. 



It is useful to cover the glass supports at their points of contact 

 with gum-lac, which is an excellent insulator. 



Glass plates are now in more general use than cylinders for the 



reduction of electricity by friction. [ELECTRICAL MACHINE.] Peihaps 



tie most perfect apparatus for producing electricity, and also measuring 



,s quantity, is that employed by Sir W. Snow Harris, and described 



his ' Rudimentary Treatise on Electricity.' 



Wo shall now observe, as our first phenomenon, that neither the 

 ylinder which has generated and given out electricity, nor the con- 

 uctor which has acquired it, exhibits the least alteration of weight, 

 or will the greatest possible accumulation of electricity produce the 

 east perceptible alteration in this way. Those who suppose that 



