222 O. N. Rood on the Electric Spark. 
The apparatus for throwing the spark on the sensitive plate 
R’ 
is seen in the diagram: a brass To y 
terminated by a freshly polished brass ball 
six-tenths of an inch in diameter, is sup- 
ported over the centre of the sensitive plate 
and insulated in the manner seen in the 
wood cut (fig. 1): the rod is graduated and 
held in position by a binding screw. The 
sensitive plate is supported on a silver disc, 
which is in metallic connexion with the rub- 
ber of the machine; a weak spring of pla- 
tinum foil rests on the collodion film and 
connects it with the silver disc; R is con- 
nected with the prime conductor. @ 
The manipulation was as follows: a glass plate three_inches 
square was carefully cleaned, coated with collodion, and sensi. 
tized in a bath of nitrate of silver of 40 grains to the ounce 
water: the plate on being taken from the bath was held in 0 
upright position, so that it could drain for one or two minutes; 
it was then placed on the stand, and the machine very — 
turned, until an apparently single discharge had been effec 
when the plate was removed about three-tenths of an inch and 
the operation repeated until twenty sparks had fallen upon Jt 
The plate was then developed and fixed as usual ; after each & 
periment the brass ball was wiped to remove dust, &. 
The pictures produced in this way are apt to be too intense, 
whereby the interior details are often obscured. Some carem en 
therefore be taken in the selection of a collodion; that which’ 
suitable for ‘‘ ambrotypes” is to be preferred: the following for 
mula answered well in my hands. 
Plain collodion, —- - - - - 802. _ 
Todid of potassium, i : ij - 40 grains. 
Bromid of ammonium, - m Ps - 20 do. 
_ This collodion should be used while still new, as it then is seD” 
sitive and not too intense in its action. 
Form of the positive spark when drawn from the prime conductor by 
a short thick metallic rod. 
_ The positive electrical spark under these circumstances a 
sists of a combination of two figures, viz.: a star and one ee 
more rin The relation which they hold to each other 1s ™ ce 
ified by the distance the spark travels in the air, that is by ed 
tension of the electricity: the two figures are usually arrane’, 
with ac nsiderable degree of symmetry. The byt Te aie 
difference in these two components, and the fact that the an? 
Pall show, cis ; «sal brush, 
form is, as I shall tie of the electrical 
