xxii MINUTES. 



A letter was received from Prof. V. M. Slipher accepting his 

 election to membership. 



The John Scott Medal and Premium were presented to Madame 

 Curie by the Board of City Trusts on behalf of the City of Phila- 

 delphia, Trustee. 



Madame Curie, in a verbal tommunication, briefly described one 

 of the instruments used in accurate determinations of radioactivity. 

 It depends on the use of the Quartz Piezo-Electrique, devised by 

 P. Curie and his brother in the early eighties. 



The essential part of this instrument consists of a plate of quartz 

 which is cut in a special manner. When this plate is placed under 

 tension, there is a liberation of electricity equal in amount, but 

 opposite in sign, on the two sides of the plate. The plate is hung 

 vertically and weights are added to the lower end. The two faces 

 are normal to one of the binary axes of the crystal. The tension 

 must be applied in a direction normal to the optic and electric axes. 

 The two faces are silvered, but the main portion of the plate is 

 electrically insulated by removing a narrow strip of the silvering near 

 the upper and lower ends of the plate. One side of it is connected 

 with the electrometer and with the conductor the rate of leak of 

 which is to be measured. The quantity of electricity set free on 

 one side of the plate is accurately given by a well-known formula 

 and depends upon the dimensions of the plate and the tension 

 applied. 



Madame Curie stated further that this instrument was used quite 

 early in her measurements, and in an improved form is considered 

 most accurate for such purposes at the present time. 



It was ordered that as a souvenir of this evening the President 

 be authorized and instructed to present to Madame Curie one of the 

 Franklin Medals struck by the order of the Congress of the United 

 States in commemoration of the 200th Anniversary of the Birth of 

 Benjamin Franklin. 



The President thereupon presented the medal to Madame Curie, 

 who accepted it with thanks. 



