MAGNETISM AND ELECTRICITY. 155 
appearing under the same conditions in the form of a simple star or 
luminous point. 
Various interesting experiments may be performed by interrupting the 
continuity of a conductor, thereby causing the electricity to leap through a 
non-conductor, as air, and thus exhibit itself in the form of a spark. The 
first of these here to be mentioned is the lightning plate (pl. 20, fig.42). This 
consists of a pane of glass, with strips of tin foil pasted upon it, as in the 
figure. so as to form a continuous communication between @ and z. The 
tin foil is then cut through, or pieces cut out of it, the cuts representing 
letters, figures, &c. At each point where the continuity is thus interrupted, 
a spark will be visible on passing the charge of a Leyden jar from z to a. 
lightning tubes are constructed on the same principle, except that small 
lozenge-shaped pieces of tin foil ( fig, 48) are pasted on tubes passing spirally 
round in a continuous line. Holding one end of this tube to the prime 
conductor of a machine in active operation, a constant series of sparks 
will be observed, answering to the points of the several lozenges of foil. 
_ Not the least interesting phenomena of electrical light are those presented 
by its passage through a total or partial vacuum. For this purpose we may 
use a straight tube of an inch or two in diameter, or an ellipsoidal glass 
vessel, as in pl. 22, fig.'71. This has metal caps at each end, one of them 
provided with a stop-cock and screw, for attachment to an air-pump, the 
other with a stuffing-box, through which slides a brass wire terminated by 
a ball. There is also a ball projecting inside from the opposite cap. On 
exhausting the air, and bringing one of the brass caps into contact with an 
excited prime conductor, and the other with the earth, a diffused violet or 
purplish light will be found to pervade the tube, passing from one ball to 
the other. If some air be admitted, the light will be in the form of 
purplish ares. Similar phenomena occur in the Torricellian vacuum. 
Experiments have been instituted by Wheatstone, with the assistance of 
a mirror rotating on a vertical or a horizontal axis, to determine the 
duration of the electric spark, as also the velocity with which electricity is 
transmitted along conductors. ‘To ascertain this latter point he made use 
of the following arrangement :—NSix balls, a, b, c, d, e, f (pl. 20, fig. 45), 
were attached in a horizontal line to a board about three and a half inches 
in diameter, called the spark board. A communication was established by 
a wire between a to the inner, and from f to the outer coating of a Leyden 
jar; b and a, d and c, e and f, were about one tenth of an inch apart; a 
coil of wire conducted from 6 to c, and another similar one from d to e. 
The length of each interval of winding between b and ¢ and d and e, 
amounted to one fourth of a mile. When the inner and outer coatings of 
the Leyden jar were brought into communication by the simultaneous 
contact of the wires attached to a and f, three sparks would be transmitted : 
one between a and b; one between c and d; and one between e and f. 
At a distance of ten feet from the spark board, and at an equal height with 
it, the apparatus with the rotating mirror was attached, its axis of rotation 
horizontal and parallel to the line of the six balls. The observer is to be 
placed with the axis of rotation opposite to him, looking down from above 
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