BEFORE COMPLETION OF THE VOLTAIC CIRCUIT. 
185 
Fig. 2. A represents the wooden base of the instrument : BB'are two glass pillars about 
three inches high, on which are fixed two brass balls with binding screws attached, b V, 
and by which discs or points of platinum, or small pieces of charcoal, may be attached : 
C is a wooden slide with an ivory scale divided into tenths of an inch, and by which 
the pillar B can be moved ; D a metal slide (to which the pillar B' is attached) with 
an index and scale to tenths of an inch ; and E a scale attached to the base of the 
instrument, graduated to ^th of an inch : the metallic slide D is moved by a micro- 
meter -screw, F, to the circumference of which is attached a scale divided into 100 
ecpial parts. One complete revolution of the screw F acts on the metallic slide D, 
and consequently on pillar B', to the extent of ^th of an inch ; and as the circum- 
ference of the scale attached to the screw is divided into 100 equal parts, every division 
will indicate an action on the pillar B' to the extent of g^th of an inch. For the pur- 
pose of reference, I will designate this instrument as the Micrometer Electrometer. 
13. It will be seen by the preceding description of the instrument, that I was now 
enabled to measure a distance of g^th of an inch, with what correctness the fol- 
lowing experiment will show. A magneto-electric machine (made to avoid the use 
of mercury, by having a metallic break attached to the armature, on which contact is 
broken, and where of course the usual spark appears as the armature is rotated,) 
was placed on the table; wires were then attached to the pillars B B' by the binding 
screws b V of the micrometer electrometer (12.), and these wires being also attached 
to the magneto-electric machine in such a manner as to form part of the circuit, on 
rotating the armature, a brilliant spark was invariably obtained on the break when- 
ever the points or balls of the micrometer electrometer (12.) were in contact, but when 
separated to the g^th of an inch, as denoted by one division of the scale, the spark 
immediately disappeared ; showing that when the circuit was divided by this minute 
space, a space not appreciable to the eye even when assisted with a powerful lens, the 
spark could not be obtained. By this experiment, I not only ascertained that the 
magnetic spark could not pass through the space of g^th of an inch, but, what was 
more important to me for the object I had in view, I proved the correct action of 
MDCCCXL. 2 B 
