SCIENCE-GOSSIP. 203 
This tube has a small hole about 3 to 4 mm. diameter 
at the bottom, the interruptions taking place at this 
The adjust- 
ment is made by raising or lowering a vertical glass 
hole, and not at the anode, as before. 
Bunsen Cert—Rounp Form. 
Fig. 11. 
rod, whose drawn out lower end works in or out of 
the hole and so alters its effective diameter. It is 
claimed for this break that the fatigue observe.l in the 
Wehnelt form, is to a certain extent avoided. 
SouRCES OF CURRENT. 
As may be easily understood, the strength of cur- 
rent employed is entirely dependent upon the size of 
the induction coil. Three sources of current are ob- 
tainable : (1) primary batteries, (2) secondary batteries 
or accumulators, (3) electric lighting circuits. Which 
of these three is employed will depend upon the size 
of the coil, and upon the locality in which the work 
is to be carried out. Each is most useful under 
certain conditions. For instance, the last two are 
barred to a worker in India or the Colonies ; or even 
in some parts of England, where a central lighting 
Station is not within his reach. On the other hand, 
to one having an electric lighting installation in his 
house, these two last sources will prove the least 
troublesome and at the same time the most economi- 
cal. These three sources we shall now consider. 
PRIMARY CELLS. 
The two chief objections to primary batteries for 
this work are, that they entail a considerable amount 
of trouble to set up every time they are used, and 
that most of them give rise to irritating fumes. X-Ray 
work sometimes lasts for half-an-hour to an hour 
without much interruption, and it is necessary that 
the battery should give a constant current during that 
time. Generally speaking, however, it is those 
primary batteries emitting objectionable fumes that 
are most constant. The three chief forms in use are 
the Grove, the Bunsen, and the Bichromate batteries. 
These are depicted in figs. 10, roa, 11, 11a, and 12 
respectively. 
The Grove cell consists of a plate of zinc bent into 
a U form, fig. r1@, placed in a rectangular porcelain 
ot, a porous pot lying in the bend of the zinc, and a 
piece of sheet platinum placed inside the porous pot. 
The outer pot is filled with dilute sulphuric acid of 
strength equal to one part of acid by volume to nine 
parts of water, the inner pot with strong nitric acid. 
The current emerges from the cell at the platinum 
electrode. Figure 10 shows eight of these cells, 
coupled up in series ; that is to say, a platinum of one 
cell connected to a zine of the next, and so on. The 
electromotive force (E.M.1¥’.) of a Grove cell is 
about 1.9 volt, and the current obtained is constant 
for a fairly lengthened period. When the cell is 
working, however, the nitric acid is reduced, and it 
is the resulting nitrous fumes that are objectionable. 
The Bunsen cell is of similar construction to the 
Grove pattern, excepting that a carbon plate is 
substituted for the very expensive platinum one; the 
Fic: 1a. Bunsen Cett-—Frar Foro. 
action being the same. Figure 11 shows the round 
form and fig. 11a the flat form of this cell. The 
E.M.F. is about the same as that of the Grove, and 
the cell is a constant current one, but nitrous fumes 
are also produced. 
A less objectionable, but at the same time a useful 
type, is the Bichromate cell. The electrodes are 
Fic. 12. BicHROMATE BATTERY. 
carbon and zinc plates. These are placed in a 
solution of chromic acid or in one of strong sulphuric 
acid and potassium bichromate, strength about one 
part of acid to five of bichromate, by volume. 
Figure 12 shows a set of five of these cells coupled up 
H 4 
