290 Rood — Experiments on High Electrical Resistance. 



failed to affect the electrometer, but with the electroscope it 

 was found that the charge had extended over 15 cm . When 

 twenty-four threads of this silk, half a meter in length, were 

 used, the electrometer gave slow strokes needing three minutes 

 each. A single thread of this silk was heated in a bath of one 

 part gutta perch, three parts rosin ; a large quantity of vapor 

 bubbles escaped, and it became a good non-conductor. When 

 it was connected with the Ley den jar, charged positively to 

 12,000 volts, not only did it not conduct, but became nega- 

 tively electrified, with a potential of about 200 volts. Negative 

 electrification under such circumstances will be referred to 

 later on. 



Paraffin paper. — Ordinary commercial paraffin paper, viz : 

 writing paper which had been passed through melted paraffin 

 wax, conducted quite well. A strip a meter in length, in fif- 

 teen minutes had acquired over its whole length a potential of 

 about 12,000 volts. Some measurements were made of short 

 strips of this paper provided with metallic armatures, a poten- 

 tial of only 110 volts being employed. 



Length. 



Breadth. 



Resistance in megohms. 



25 mm 



lg mm 



80,000 



63 mm 



a 



125,000 



132 mm 



U 



300,000 



It is evident that the resistance of the connecting arma- 

 tures has influenced these results. 



Mica. — Using a potential of 12,000 volts, a strip of mica 

 (muscovite) 30 cm long and 2 cm broad, was compared with a strip 

 of paraffin paper of the same dimensions ; the resistance of the 

 mica was sixty times that of the paraffin paper. The hygrom- 

 eter at the time indicated 30 per cent of moisture. Two strips 

 of mica from the same specimens were prepared ; the length 

 of each was 27 cm the breadth 2 cm . Both were heated till their 

 edges had become somewhat white, and on the following day 

 their relative resistances were determined ; these turned out to 

 be nearly as 1 to 2. Knowing that mica when heated gives off 

 traces of water and fluorhydric acid, a piece of this mica was 

 heated in a Bunsen flame till its structure was destroyed, and 

 on the following day, having been provided with a tin-foil 

 armature, it remained for twenty minutes in contact with a 

 Leyden jar charged to a potential of 12,000 volts. It was then 

 removed and presented to an electroscope capable of indicat- 

 ing 100 volts, but failed to show any charge. 



In the case of all the substances that thus far have been men- 

 tioned except two, it was found that when they were connected 

 with the Leyden jar, that within at least fifteen minutes the 



