.
.
_
‘
_
l_}H
_H
\
Ÿ
r
"
Equipment ;
_
fact already been d0ne‘ in
book?) This advice “is soündand thor—
oughly recemmended, but it is a factthat When a person buys, say, a
‘
heater and a thermostat he does not oftencall in an electrician to
“
wire up the circuit, but for various reasons does it himself. Having ‘
Wired upŸa heater system which seems satisfactory, confidence is
gained" for more complex enterprises often with disastrous results.—
,
ltis therefore felt that it would be better to explain some of the ‘.
required electrical theory in simple terms and to discuss some simple
,
methods, rather than to dismiss the subject with advice which,
although good, willprobably not be heeded, As an introduction,
Figure 36 shows the electrical connections for a typical aquarium.
__
The nominal mains electricity supply in the British Isles is 240 V
a.c. at 50 Hertz. The two letters ‘a.c.’ stand for ‘alterhating current’
and show that the supply uctuates in a cyclic fashion. A battery,
"
for instance, supplies direct current (dc) which is continuous, and
has a positive and a negative terminaL An a.c. supply. «alternates
’
‘
between positive and negative at its terminals, that is one terminal is
_
positive at one, instant, négative at the next, then positive'again, and
’ _
so on, and the other terminal is always in the opposite condition.
-
,
quency of the supply and is
cycles per second, now_adays _.
_
Called Hertz (Hz)
"
‘
_
.
_
Thé voltageof the supply, 240 V in Britain, is the “pressure’ of the
electriCity which is available andwaitihg to push the electric current
_
into any load
on the supply ”system. Thus the designation
‘
thé "electridty available
the -/supply pOint‘ is at 240 vand
between positive and negative at a raté…of 50 »
second.
_
’
“
-
_
'
_
COnn‘ected‘iscall_
current
in amperes,‘usually
f
thé"
«that appliariÇÏe, Which is
}
Because the
of current
a;ÿ_ :
.
drïload
de“p end “also“ onthepressure of
,
volt rj{;
…î —_—î gage availablë “imp uSh
appliance,«;_’thclÏh« is a. Simple relat10n- Ï L
sh1pbetweenthesethreeproperueswhmhlsexpressedas
:
Ï
power=CufrentXvoltage
-‘
—
or
‘
Therearefurtherœmphcaüonstotheaboveequatwnmmre—
extremelÿhlghcurrentvaluesaredrawnbutthesed0notconcern
H um…‘m … ‘“…"u’M l\ l mm
… m …= m -…
m M
< :«“—
.
_
‘
_
l_}H
_H
\
Ÿ
r
"
Equipment ;
_
fact already been d0ne‘ in
book?) This advice “is soündand thor—
oughly recemmended, but it is a factthat When a person buys, say, a
‘
heater and a thermostat he does not oftencall in an electrician to
“
wire up the circuit, but for various reasons does it himself. Having ‘
Wired upŸa heater system which seems satisfactory, confidence is
gained" for more complex enterprises often with disastrous results.—
,
ltis therefore felt that it would be better to explain some of the ‘.
required electrical theory in simple terms and to discuss some simple
,
methods, rather than to dismiss the subject with advice which,
although good, willprobably not be heeded, As an introduction,
Figure 36 shows the electrical connections for a typical aquarium.
__
The nominal mains electricity supply in the British Isles is 240 V
a.c. at 50 Hertz. The two letters ‘a.c.’ stand for ‘alterhating current’
and show that the supply uctuates in a cyclic fashion. A battery,
"
for instance, supplies direct current (dc) which is continuous, and
has a positive and a negative terminaL An a.c. supply. «alternates
’
‘
between positive and negative at its terminals, that is one terminal is
_
positive at one, instant, négative at the next, then positive'again, and
’ _
so on, and the other terminal is always in the opposite condition.
-
,
quency of the supply and is
cycles per second, now_adays _.
_
Called Hertz (Hz)
"
‘
_
.
_
Thé voltageof the supply, 240 V in Britain, is the “pressure’ of the
electriCity which is available andwaitihg to push the electric current
_
into any load
on the supply ”system. Thus the designation
‘
thé "electridty available
the -/supply pOint‘ is at 240 vand
between positive and negative at a raté…of 50 »
second.
_
’
“
-
_
'
_
COnn‘ected‘iscall_
current
in amperes,‘usually
f
thé"
«that appliariÇÏe, Which is
}
Because the
of current
a;ÿ_ :
.
drïload
de“p end “also“ onthepressure of
,
volt rj{;
…î —_—î gage availablë “imp uSh
appliance,«;_’thclÏh« is a. Simple relat10n- Ï L
sh1pbetweenthesethreeproperueswhmhlsexpressedas
:
Ï
power=CufrentXvoltage
-‘
—
or
‘
Therearefurtherœmphcaüonstotheaboveequatwnmmre—
extremelÿhlghcurrentvaluesaredrawnbutthesed0notconcern
H um…‘m … ‘“…"u’M l\ l mm
… m …= m -…
m M
< :«“—