204 
in series and fitted into a frame provided with a lifting 
arrangement, whereby the plates are raised out of the 
liquid when the battery is not required. The 
Bichromate cell entails less trouble than, but is not so 
constant as, either of the Its 
E.M.F. is about 1.9 volt. 
Not more than four of any of the above cells, con- 
nected in series, should be necessary for a properly- 
onstructed induction coil giving a normal spark of 
preceding forms. 
a 
“ 
ss 
Sreconpary BarrERY—PorRTABLE Form. 
FIG. 13. 
three to four inches, while six cells should be 
sufficient for a six’ or eight inch spark. For coils 
above this size, it is not very often that primary 
batteries are used, unless the other sources of current 
are not available. 
SECONDARY CELLS. 
Secondary cells differ from primary ones both in 
construction and action. They consist of two or 
more lead plates, either plain or covered with lead 
sulphate paste according to the particular make, 
immersed in dilute sulphuric acid. If the two 
surfaces, facing each other, of any two plates are in 
the same condition, there is no difference of potential 
between them ; and therefore no current is obtainable 
from the cell. A “charging” current, from an 
external source, is sent through the cell when peroxide 
of lead (PbO.) is formed upon the surfaces of the 
plates connected to the positive terminal of the 
charging current, while spongy metallic lead or oxide 
of lead (PbO), is formed upon the others. It is in 
consequence of this difference in chemical condition 
between the plates, that an E.M.F. is set up; 
and that the cell is capable of producing 
a current when required. While the cell thus dis- 
charges itself in giving out a current, the plates 
return again to their original inert condition, when 
they will require a fresh current to recharge them. 
The E.M.F. of a secondary cell is about 2.2 volts, 
and is very constant almost throughout the 
whole time of discharge. The positive ter- 
minal of the cell is the one to which the 
positive terminal of the charging current is connected, 
and whenever the cell is to be recharged, care must 
be taken that these same connections are made, 
otherwise the cell will become ‘‘ unformed.” It 
should, moreover, be remembered that the E.M.F. of 
the charging current must not be more than 2.5 to 
SCIENCE-GOSSIP. 
3 volts per cell, or the life of the latter will be con- 
siderably lessened. 
The capacity of any secondary battery, that is the 
amount of electrical energy obtainable from it, will 
depend upon the size and number of the plates in 
each cell, and also upon the particular type of cell. 
This capacity is generally defined in ampere hours : 
that is, the number of hours the battery, when 
charged, is capable of producing a current of I 
ampere. For instance, a 60-ampére-hour battery 
will give I ampere for 60 hours, 2 amperes for 
30 hours, 4 for 15 hours, and so forth, as far as the 
limit of constancy guaranteed by the maker. 
Secondary batteries for X-Ray work are generally 
portable, as is shown in fig. 13. This particular 
form, as will be seen, consists of six cells, producing 
in all, an E.M.F. of about 13 volts. Its usefulness 
is much increased by having an additional terminal 
connected to each cell, so that intermediate values 
of E.M.F. may be used, if required. Such a battery, 
if say of 45 ampere-hours capacity, measures about 
18x88 inches and weighs about 5olbs. 
Before leaving the question of secondary batteries 
one important point must be mentioned. When a 
battery has just been fully charged its rate of dis- 
charge is much above the normal, and great care 
should be taken when connecting it direct to an 
induction coil, lest the latter becomes damaged by 
the heavy current. A resistance should be inserted 
for safety, and which can be cut out again when the 
discharge rate is normal. As the smaller induction 
coils, from 3 to 6in. spark, require only about 6 to § 
volts to work them, 3 or 4 secondary cells would be 
ample. For larger coils the necessary E.M.F. in- 
creases up to 16 volts or more. 
(To be continued.) 
CAMARA PERTANA.—Dr. Pertana, chief of the 
Bacteriological Institute at Lisbon, has fallen a 
victim to the plague, while studying its ravages at 
Oporto. He was a most ardent and accomplished 
bacteriologist, though his works being written in 
Portuguese, are not much known. It was his verdict 
on specimens sent to him from Oporto, that estab- 
lished the existence of plague in that city. At this 
time, when every worker investigating this bacillus 
is so urgently needed, Dr. Pertana’s death is the 
more to be deplored. 
To Finp puE SoutH.—The ‘‘ Photogram ” publi- 
cation is issuing some cards useful to amateur 
photographers and others. One before us shows 
how to find due South without a compass. Hold an 
ordinary watch horizontally, so that the hour hand 
points to thé sun. The position half-way between 
that point and the XII (12 o’clock) pot of the 
watch, is due South. Thus: at 4 p.m., when the 
hour hand points to the sun, the dial point IT (or 
2 p.m.) is due South. Or, at 6 a.m., if the hour 
hand points to the sun, the dial point IX (or 9 a.m.) 
is due South. Count forward in the moming, back- 
ward in the afternoon. Facing the South, East is 
always three hours earlier ; West is three hours later. 
North is the opposite side of the dial. The rule is 
based on the fact that the hour hand makes two 
complete circuits of the dial, while the sun is making 
one apparent circle round the earth. 
