270 M. Becquerel on the Develop7nent 



when they possess that property : the surface which is to be 

 subjected to pressure is then rubbed with alcohol to remove 

 any greasy matter which might be found upon it, and the body 

 is suffered to remain for some time in the dry air of the ba- 

 lance : by this means the small stratum of moisture which 

 usually adheres to the surface of all bodies is removed. As to 

 the substances which are not mineral, it is sufficient to deprive 

 them of their hygrometrical water. This latter precaution is 

 indispensable ; for certain substances, as, for instance, crystal- 

 lized sulphate of barytes, mica, sulphate of lime, &c., only be- 

 come electric by pressure in proportion as they have been 

 previously dried. 



At the moment in which the two bodies are placed upon 

 each other, the greatest care must be taken that they undergo 

 no friction, since that would produce a complication of effects 

 for which it would be impossible to account. This is effec- 

 tually prevented by placing the bodies in such a manner, that 

 the beam of the balance experiences no oscillation in any di- 

 rection. For this purpose a vertical stem is fixed to the foot 

 of the balance, which rises and falls by means of a rack and 

 nut, the rack terminated at its upper extremity by a fork in 

 which is placed one of the arms of the beam. This rack and 

 nut is so placed that the beam can rise and fallwithout lateral 

 oscillations. Further, to be quite certahi that the slightest 

 friction has not influenced the quantity of electricity arising 

 from the pressure, the pressure is allowed to continue for some 

 time *. 



It is very difficult to determine the law of electric intensities 

 caused by equal pressure and unequal degrees of velocity of 

 separation ; the researches which that law demands cannot be 

 made with the apparatus we are using, but we easily find the 

 law of the electric intensities which result from different pres- 

 sures and velocities of separation giving the maximum of 

 effect. Let us suppose then, that by the preliminary experi- 

 ments we have determined the maxima velocities, and let us 

 see what happens when the pressures increase. Let us take 

 different substances and press them all with the same disk 

 of cork. 



* In the experiments, the object of which is to determine the relation 

 between the electric densities and the corresponding pressures, we must 

 not subject to pressure substances in which sHght alterations of the sur- 

 faces pressed would occasion great difterences in the quantities of electri- 

 city developed. For instance, cork and elder-pith are improper, because 

 the smallest change of temperature in either of these bodies is frequently 

 sufficient to modify considerably the development of electricity; such 

 bodies as sulphate of barytes and cork must be taken. 



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