640 Prof. A. Battelli and Mr. L. Magri on 



of calorimetric readings, requiring always a satisfactory 

 accord between the two sets of readings. 



We next obtained the absolute value of the discharge- 

 potential by standardizing our electrometer with a good 

 absolute Kelvin balance-electrometer made especially for 

 potentials of the order of magnitude of those used by us. 



Acccording to the potential- difference we had to measure, 

 the electrometer was regulated so as to give to its constant 

 the value 1*505 or the value 2*71. 



Galvanometer for measuring the Residual Discharge. 



41. But in order to know exactly the portion of energy 

 really intervening in the discharge, it was necessary, as 

 above stated, to determine also the value of the residual 

 charge. 



For this purpose the condenser was charged by means of 

 a large double Holtz machine, the motion of which was 

 maintained slow and uniform by means of an electro- 

 motor. When we wished to determine the residual charge, in 

 order that the condenser might not be charged very quickly, 

 in most cases we approached a pole of the machine itself 

 terminated by a point to a plane electrode connected with 

 one of the armatures of the condenser, instead of establishing 

 a direct connexion with the machine, the other armature 

 being joined to earth. 



After ascertaining in the required manner the distance 

 between the electrodes of the spark-gap, the velocity of the 

 motor was modified so as to have sparks passing in the spark- 

 gap at intervals of about one or two minutes. After this 

 we proceeded to measurements ; that is, we set free a short 

 pendulum conveniently placed, as soon as the first spark had 

 passed in the spark-gap, so as to establish the connexion 

 between the condenser and one of the terminals of a ballistic 

 galvanometer, the other terminal being joined to earth. 



In order to secure a good insulation and a complete pro- 

 tection of the needle from the electrostatic actions, the frame 

 of the galvanometer was formed of only two layers of wire 

 covered with a thick guttapercha coating, and the needle 

 (made up of a small magnetized steel mirror) was inclosed 

 in a cylinder, whose lateral surface was of metallic gauze, 

 the bases being of brass. Readings were effected through a 

 narrow slit made in one of the bases. 



Jn our experiments, where sparks never exceeded 5 mm. 

 in length, and where the resistance of the circuit was ex- 

 cessively small, the residual charge, being due only to the 



