ELECTRICITY. 



Electricity can be transferred or communicated from one body to another. 

 An electrified ball can be deprived of its electricity by being touched with a 

 metallic rod, but if we touch it with glass or wax the electricity will remain 

 unaffected. Hence, metals are said to be conductors, and glass and wax non- 

 conductors. Bodies differ greatly in their power of conduction. A list of the 

 principal substances that possess these properties will be found on the 

 diagram. The most convenient mode of obtaining an accumulation of elec- 

 tricity, is by employing a cylindrical glass jar, coated within and without nearly 

 to the top with tin-foil, and having a cover of baked wood incased with sealing- 

 wax to exclude moisture and dirt. A metallic rod rising above the jar, and ter- 

 minating in a brass nob, is made to descend through the cover and communicate 

 with the interior coating. An apparatus of this kindjs called a Leyden jar, and is 

 represented at fig. 6. 



Fig. 7 represents a discharging rod, for establishing a communication between 

 the inner and outer coating of the jar. The handle is of glass, to prevent the 

 operator from receiving the charge of the jar. 



By uniting a sufficient number of jars, we are able to accumulate a large 

 amount of electricity. Such a combination of jars is called an Electrical Battery. 

 (fig. 8). 



Fig. 9 represents an instrument called a Universal Discharger, which is used 

 for passing charges through any substance that may be laid on plate A. 



Fig. 10 represents an Electro-meter, an instrument used to detect the presence 

 of electricity. If charged, either by placing the instrument on one of the con- 

 ductors of a machine, or on the rod of an electrical jar, the reed rises and marks 

 as an index on the graduated semicircle the angle of divergence, by which the 

 comparative amount of electrification may be estimated. The hairs upon the 

 well-known electrical toy, represented at fig. 11, are spread out, and stand on end 

 upon the same principle. 



Fig. 12 represents two bells suspended from a brass wire, D i>, supported by 

 a glass pillar, A. The electricity being conducted to the knob E, passes down 

 the wires, D D, to the bells, which are then positively electrified, and attract the 

 clappers, c c, that are negatively so, in consequence of being insulated by silken 

 strings. The clappers having become charged, strike against the centre bell to 

 discharge themselves, and thus a peal is rung on the bells till the electricity is 

 taken off. 



Electric sparks are of a bluish colour in the atmosphere in its ordinary state, 

 und their character depends almost entirely on the form, area, and electrical 

 intensity of the discharging surfaces. If the small ball, p (fig. 13), be attached to 

 the prime conductor of a machine, and a larger ball, N, be presented to it, a 

 series of brilliant sparks, of a crooked or zigzag form, will pass from the smaller to 

 the larger ball. When, however, electricity is given off from short points, it 

 is unaccompanied by noise, and presents the brush -like appearance represented 

 at fig. 14. 



The influence of pointed conductors on electrically charged bodies, was first 

 observed by Franklin, who showed that when presented to them, their charges 

 became silently and rapidly dissipated. Hence, the utility of pointed conductors 

 to secure buildings from the effects of lightning (fig. 15). 



Galvanism- The production of electricity in this case arises from the cor- 

 roding action of an acid upon metallic surfaces. The acid being interposed be- 

 tween two plates of dissimilar metals, usually copper and zinc, and the zinc 

 being the more oxidable, gives out positive electricity. The two plates are con- 

 nected together at the top by a wire, and this communciation establishes what is 

 called a voltaic or galvanic circuit ; the electricity circulating round the zinc, 

 wire, copper, and liquid (fig. 16). 



There are many fortes of galvanic batteries. Fig. 17, represents Daniell's 

 battery, consisting of a cylinder of copper containing a porous tube, having a 

 solid rod of amalgamated zinc in its centre. Fig. 18, represents Griffin's im- 

 proved Smee's battery, consisting of six cells ; the plates are arranged upon a 

 frame suspanded, by means of a rod and rocket wheel, over the trough con- 

 twining the exciting liquid. By this arrangement any desired degree of powei 

 may be obtained by merely raising or depressing the frame. 



