280 



tried, and, as he writes me, it succeeds. The same (Mr. Henley) 

 has invented an electrometer, which seems useful. I send you a 

 draft of it. It shows in what degree a bottle is charged ; that is, 

 whether half, three-quarters, &c. : so that knowing the force of a full 

 chai-ge of any bottle or battery, you may by this, while charging, 

 know the proportion you have of such force. Your experiment, 

 showing that a stroke with black lead on paper would conduct a 

 shock, was new to me. I mentioned it to some, who since tell me 

 that they also find the solid black lead in a pencil conducts as well as 

 wire; which, indeed (the other being true), is not to be wondered at. 

 It is, however, the only property of metal black lead possesses, as far 

 as we yet know it. Mr. Canton melts silver and gold wire by elec- 

 tricity, not only into fine white little globules, but ^Iso into spherules 

 of glass, some of which he has shown me by his microscope. They 

 were transparent, the light passing through them, and appearing in a 

 focus on the paper. Mr. Henley has several times melted iron wire 

 lying at the bottom of a white stone plate filled with water. The 

 iron was destroyed, and marked the plate with an indelible black 

 stroke. Sparks flew from it out through the water, and fell red-hot 



j) mP 



A B, an ivory rod, round, with a knob at the top, six inches high. 



C, a short tin socket, fixed to the prime conductor, to receive the end of 

 the iron rod. 



D, a cork or pith ball, at the end of a small ivory arm, turning on an axis 

 atE. 



F, a semicircular plane of ivory, graduated at the edge, to mark the rise of 

 the ball by the small arm passing over the graduations. 



