846 Dr. Joseph Reade on Electro- Rotatoty Points, 



repulsion. The oscillations are then explained. At C the bar 

 becomes electro-positive and at B and C electro-negative; final- 

 ly, the bar fixes at G w C perpendicular to K. Although sta- 

 tionary, we are not to infer that all electricity has been dissi- 

 pated, for to my surprise, on holding the point of my fore finger 

 at O perpendicular to the bar, C was repelled to B. Here 

 my finger must have become electro-negative, the same as the 

 needle; when my finger was held at n the bar was attracted; 

 from this we are not to infer any electro-polarity, for my finger 

 must have changed its electricity with the distance ; on substi- 

 tuting an excited stick of sealing-wax instead of the glass tube, I 

 found that the needle was likewise repelled, according to the re- 

 ceived opinion, that excited sealing-wax is always electro-nega- 

 tive; in this case the bar and also my finger must be electro- 

 positive by induction. However, I shall not stop to theorize 

 on these interesting facts, for facts in our present state of ex- 

 perimental philosophy are more valuable than theories; for- 

 tunately inductive reasoning is gaining ground, and the wild 

 dreams of the greatest men melting away, and only an ipse 

 dixit is at a discount. 



For some time I have used this brass needle as a revolving 

 electrometer for measuring relative electricities, insensible to 

 either the gold-leaf or torsion electroscopes, for it is a mis- 

 nomer to call them by any other name ; I used it in the follow- 

 ing manner: — Having surrounded the insulated stand with a 

 graduated semicircle, it was placed parallel to the substance 

 to be examined, and then, if the electricity was very weak, the 

 arm was moved to 45° ; on removing the hand the needle os- 

 cillated, and the number of oscillations indicated the relative 

 force. When the bar was stationai-y, by repelling with the 

 point of my finger I could ascertain the most minute quantity. 

 Let us now apply this revolving electrometer* to an entirely 

 nevi' subject ; — the pyro-electricity of glass, considerably 

 greater than that of the tourmaline. 



Experiment 2. — On the Pyro- Electricity of Glass. 



October 10, 1844. — "When the thermometer was at 64° with 

 the door of the parlour open, I placed the needle on its insu- 

 lated stand on the hob, and then placed two tumblers, one at 

 each side, so as to allow the lever to revolve; the needle 

 immediately began to revolve at the rate of sixty or a hun- 

 dred, according to the heat, in a minute, and continued the 

 motion as long as the fire was sufficient. When the tum- 

 blers were removed, the revolutions decreased and finally 



♦ This iBstrument may be seen at Messrs. Watkins and Hill's, Charing 

 Cross. 



