526 
Mr. Babbage on electrical 
greater, and it may retain it much longer than the screen, 
and in this case we ought to anticipate a motion of the needle 
in the same direction as the plate. 
Let us now examine the case of one of the bodies possess- 
ing the same kind of electricity as the needle. Let the same 
letters in figure 2 represent the electricities when the plate is 
at rest ; thus, when it moves in the direction of the arrow, the 
electric curve will assume the position of the dotted line a ; 
and since it is of an opposite kind to that of the screen, it will 
attract it, and will tend to draw the electric curve B on the 
screen into the position b } b, marked by the dotted lines. In 
this case the electric forces are both on the same side of the 
perpendicular from the needle ; but since they are of opposite 
kinds, one will attract, the other will repel the needle ; and 
according to the strength of these forces, it will advance, be 
stationary, or recede. This explanation accords with the 
apparently capricious nature of the fact ; the circumstances 
on which a direct or a retrograde motion depends are so 
numerous, and in the present state of our knowledge we have 
so few data for calculating their influence, that it is not sur- 
prising that the result of any given combination should be 
uncertain. Besides the relative distances, we ought to be 
acquainted with the relative conducting powers of the screen 
and plate, — the intensity which can be excited in each by a 
given inducing source— the quantity of that inducing action 
which can be transmitted through the interstices of the 
screen — the time each body takes to acquire and give up 
electricity — before we can attempt accurately to predict the 
result of any proposed arrangement. 
There is another circumstance in which the above expla- 
