M. Melloni on a new Electroscope. 279 



the air within and the air outside the apparatus, when the glass 

 is fixed by the pressure of little screws upon the ring of metal 

 which keeps it in its place. 



The divided circle which measures the angles formed by the 

 repulsion of the index, is to be perforated in the centre to 

 give a free passage to the fixed cup A. The conducting wire is 

 contained in a tube of varnished glass, filled with mastic ; it is 

 bent twice at a right angle in the same vertical plane, so that it 

 resumes its original direction, and thus terminates at the outer 

 piece of metal intended for the introduction of the electrical charge. 



Into the space below the dial plate one or two reservoirs filled 

 with chloride of calcium are passed, by means of openings in 

 the bottom of the metal receiver, which are afterwards closed by 

 screws. The receiver is supported on a tripod furnished with 

 screws, which serve to place the suspended portion of the appa- 

 ratus in equilibrium. 



Lastly, the necessity of moving the apparatus from place to 

 place, and of giving a certain initial angle of deviation to the 

 moveable lever, renders it necessary that the upper extremity of 

 the tube in which the silk thread is enclosed should be capable 

 of motion in two directions. The first is a simple vertical move- 

 ment, to allow the inner cup to rest on the flat bottom of the 

 outer cup, and to raise it again to the proper height ; the 

 second a horizontal rotatory movement, to cause a slight devia- 

 tion of the index at the commencement of each series of experi- 

 ments. The rotatory movement will be communicated to the 

 moveable system by means of the torsion of the silk. 



As the same force is the cause of the resistance which stops 

 the electrized lever at a greater or less angular distance, its 

 strength must be proportioned to the weight of the moving 

 mass. On this account, instead of a single thread from the 

 cocoon, it is useful to employ several of them united, not by 

 twisting, but simply glued together by the action of their own 

 gummy nature and hot water. 



If the force of the torsion of the thread be found too weak, 

 and it be desired to abridge the time required for the observa- 

 tions, it is only necessary to place parallel to the index on the 

 moveable cup a little magnetized needle, as is done with the 

 indicator of Peltier's electroscope, and to arrange the appen- 

 dages of the fixed cup in a direction forming an angle of 4< or 

 5 degrees with the magnetic meridian. VV r e must not forget, 

 however, that in this case we lose in exactitude what we gain in 

 rapidity of observation. The assistance of the magnetized 

 needle may nevertheless be useful under some circumstances, 

 and especially when the great humidity of the air rapidly removes 

 the electricity to the exterior of the instrument. 



