824 



LESSONS IN ELECTRICITY. 



FIG. 50. 



ed mechanically. Introducing the wet 

 string w into the circuit, ignition infalli- 

 bly occurs when the spark passes. 



This is the place to fulfil our promise 

 to ignite gunpowder by the " hand- jar. " 

 Fig. 50 explains the arrangement. H 11' 

 are the hands of the insulated person. 

 F the hand of the uninsulated friend, i 

 (he india-rubber between both hands. 

 The lead ball B is suspended by a wet 

 string s. On the little stand P, connect- 

 ed with the earth, is placed the powder. 

 The charging of the hand- jar is described 

 in 22. When charged, it is only nec- 

 essary to bring the ball n down upon 

 the powder to cause it to explode. 



26. Electric Light in Vacuo. 



The electric light in vacuo was first 

 observed by Picard in 1675. While 

 carrying a barometer from the Observa- 

 tory to the Porte St. Michel in Paris, he 

 saw light in the upper portion of the 

 lube. Sebastien and Cassini observed it 

 Afu?r wards in other barometers. John 

 iicrnouilli devised a " mercurial phos- 

 phorus," by shaking mercury in a tube 

 vhlch had been exhausted by an air- 

 pump. This was handed to the King of 

 Prussia Frederick I. who awarded for 

 it a medal of forty ducats value. The 

 great mathematician wrote a poem in 

 noiior of the occasion. 



Fia. 51. 



Bernouilli failed to explain the effect. 

 The explanation was reserved for Ilauks- 

 bee, who in 1705 took up the subject 

 and experimented upon it before the 

 Royal Society. On tke plate of an air- 

 pump he placed two bell-jars, one over 

 the other. Tin outer and larger jar was 

 open at Hie top. Into the opening 

 llauksbee iixod, air-tight, a funnel, 

 which he stopped with a plug of wood 

 and filled with mercury, lie exhausted 

 the space between the two jars, withdrew 

 the wooden plug and allowed the mer- 

 cury to stream against the outer surface i 

 of the inner jar. lie thus obtained aJ 

 shower of lire. This is a truly beautiful 1 

 experiment when witnessed by an ob- 

 server close at hand. 



A copy of Hauksbee's own figure il- 

 lustrating this experiment is annexed, 

 fig. 51. M is the funnel containing the 

 mercury, ithe plug of wood, 8 the. outer 

 and s' the inner bell-jar. Instead of the 

 plug P, an india-rubber tube, held by a 

 clip, may be employed with advantage to 

 connect the funnel with the exhausted 

 jar. By gradually relaxing the clip the 

 mercury may be made to fall at a rate 

 corresponding to the maximum luminous 

 effect. The streams of light produced 

 are very beautiful, but they arc more 

 continuous than they arc shown to be by 

 Uauksbuo, 



In 1706 llauksbee referred the phe- 



