ELECTRIC PHASNOMENA OF THE ATMOSPHERE. 379 
that setting out from the point where an instrument, an electro- 
scope for example, has been in a state of equilibrium, (or, let us 
say, that setting out from the point where the stem has been 
put into communication with the armatures and the plate, and 
the electric state in the stem has afterwards been increased 
or diminished,) it is what it was before, plus what has been 
added to it, or minus what has been taken away from it. The 
electric sign will then depend on the starting-point of the instru- 
ment, on a point of equilibrium which may vary excessively, and 
which as much exists at the centre of an atmosphere at its maxi- 
mum of resinous tension as at the minimum of the same tension. 
6. In the course of this memoir I shall frequently speak of the 
gold leaf electroscope, although it is not the instrument which I 
employ ; it has neither the precision nor the extent of indication 
requisite when we would interrogate phenomena so variable in 
their electric tension as are those of the atmosphere. I employ 
the index electrometer which I made known (Ann. de Chim. et de 
Phys., 1836, t. lxii. p. 422) and which I -have reduced to the 
proportions of an ordinary electroscope. I have modified it only 
by curving the fixed conductor, in order to bring it back to the 
centre by prolonging it vertically two decimeters, and by sur- 
mounting it with a hollow metallic ball eight centimeters in 
diameter. I shall describe it more in detail in the work which 
will contain the applications and the tables of observations. This 
Memoir treating only of the general principle, I shall more fre- 
quently mention the gold leaf electroscope as being more known 
to the reader. 
7. We know that electrometers consist of two distinct parts ; 
one which must always communicate with the earth, the other 
which is insulated from it, and carries with it moveable bodies, as 
gold leaves, a needle suspended on a pivot, &c. The sum of 
electricity which constitutes their equilibrium may be altered in 
two ways. Ist. It may be increased by giving to the indicating 
element a greater resinous tension. 2nd. It may be diminished 
by taking from this resinous tension. In the first case the sign 
is resinous in plus, and it preserves the term resinous; in the se- 
cond case it is resinous in minus, and it bears the name of vitreous, 
although it in no way differs from the first case, excepting that 
the resinous tension is less than that of the point of equilibrium. 
8. Rheometers (multipliers) in like manner indicate only the 
difference of the contrary propagations; they are also subject 
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