Volume 33 Number 48
                 Produced: Wed Sep  6  6:24:57 US/Eastern 2000

Subjects Discussed In This Issue: 

Children in Shul
         [David Weitz]
Electricity on Yomtov (2)
         [Neil Kummer, Dovid Oratz]
Potentiometer and Yom Tov (3)
         [Yona Newman, David Charlap, Stan Tenen]


From: David Weitz <weitzd@...>
Date: Tue, 5 Sep 2000 11:44:20 +0200
Subject: Children in Shul

    I am amazed that the British institution of the Children's Service
has not become more widespread. This is essentially a shortened sevice
(about an hour or so) led by parents or responsible teenagers held in
the shul's Bet Midrash or other suitable location and gives children an
educational experience while preventing them from disturbing the
davenning in shul. The exact format varies from place to place depending
on the age of the children - in larger Kehillot there are 2 or 3 such
services for different age groups. As well as fulfilling the Chofetz
Chaim's instructions about teaching children when to answer "Amen", the
participants also learn to lein, read a Haftara and daven at the Amud in
an atmosphere less critical than the main shul, as well as getting to
hear a suitable Dvar Torah. As for the youngest children, surely creches
could be organised - either to be run by mothers in rotation or by
reponsible teenagers.

Isn't it about time this practice was adopted by Israeli shuls where, in
many cases, trying to daven with Kavannah is nearly impossible because
of the noise of children running around and parents trying to control
them.  This allows Shul going on Shabbat morning to be a positive
experience for the whole family, without the husband/father having to go
to a Hashkamah minyan so that his wife and children can go to Shul too.

David Weitz


From: Neil Kummer <kef@...>
Date: Tue, 5 Sep 2000 14:26:07 +0200
Subject: Re: Electricity on Yomtov

> I beg to differ.  There is indeed a fundamental difference
> between an electrical circuit and fluid flow

Do electrons really flow through a conductor, getting tossed from atom
to atom?  Or is it a wave where electrons change orbits in one atom,
inducing the next atoms electrons etc...

> Today we operate at lower voltages, and in copper-to-copper contact we
> use coatings and the shaping of the conductor to cut down on the
> sparking, which in some circumstances might be a fire hazard.

I understand that electronics in general and computers specifically are
heading towards extremely low voltages...soon to be lower than the
voltage transfer when you touch your hand to your face.  What then?

And about that time their will be biological machines that are half your
own biology and half very low voltage machines/computers.

Neil Kummer

From: Dovid Oratz <dovid@...>
Date: Tue, 5 Sep 2000 16:05:59 +0200
Subject: Re: Electricity on Yomtov

Mike Gerver wrote:
> I once asked a shayla about raising and lowering lights on Yom Tov, by
> using a dimmer switch, and was told that I could do it.  When I casually
> mentioned this to another rabbi a long time afterwards, he said that I
> should stick by the psak I got, but that if I had asked him, he wouldn't
> have allowed it.

There is a problem with the first Psak. Kibuy (extinguishing fire) is
generally only permitted for the sake of cooking (e.g., lowering a flame
or turning it off to prevent food from burning). At least Rav Moshe
Feinstein (and at least in his written Tshuvos) equates lowering a flame
with extinguishing a flame.  Accordingly, he would certainly not have
permitted turning a dimmer switch down on YomTov.


From: Yona Newman <yona_n@...>
Date: Tue, 05 Sep 2000 12:52:52 GMT
Subject: Re: Potentiometer and Yom Tov

Regarding Mike Gerver's question about dimmer switches.

A dimmer switch uses an electronic switch (triac or thyristor) which can
be either fully conducting (on) or non-conducting (off). Depending on
the position of the control potentiometer an integrated circuit
generates timing pulses to turn on the switch and allow through the
electricity for more or less time in a given period.

This varies the average energy supplied to the light and so the level of
illumination but does not require any (well almost any) dissipation of
energy in the dimmer.

Originally the sinusoidal waveform of the mains voltage was interrupted
for part of each cycle but this created too much interference with other
electronic equipment, so now a number of complete cycles of electricity
are allowed through, then there is a break of no current for another
number of complete cycles.

My apologies if this is over-technical.

By the way, I could raise some very interesting points about zero-point
switching (which guarantees no physical spark) and also modern embedded
operating system software (which rely on totally independent mechanisms
for detecting a change, deciding what to do about it, and actually
carrying out an action).

I wonder sometimes how up-to-date our rabbis are on advanced technology.

Another example, a shiur I went to some time again about how to kasher
utensils, when we talked about plastic, the rabbi was unaware of the the
two different forms of plastic - thermo-setting and thermo-plastic -
which is very relevant to the halacha.

Yona Newman

From: David Charlap <shamino@...>
Date: Tue, 05 Sep 2000 13:22:34 -0400
Subject: Re: Potentiometer and Yom Tov

Mike Gerver writes:
> Regarding Eitan Fiorino's question in v33n42 about what is wrong with
> raising or lowering the heat on an electric burner on Yom Tov-- I
> think there was a long post dealing with this issue a few years ago.
> From what I remember, the problem was that the burner controls on
> electric stoves do not, in fact, use potentiometers, but make and
> break discrete electrical connections, even when the control knobs
> seem to be turning smoothly.

There are two kinds of electric-stove controls.  One kind uses discrete
pushbuttons.  The other uses a knob.

In both cases, the controls do not directly control the voltage to the
burners.  Rather, they control a thermostat, which turns the electricity
on and off as required to maintain the temperature set by the control.
This dissipates much less power than a potentiometer directly wired to
the burner element would - making it more efficient and less of a fire

So, even with a knob-type control that has a continuous potentiometer-
like circuit underneath, you can't just turn the knob to adjust the

Rather, you must treat it like a thermostat.  Turning the temperature up
while the element is "on" is OK, because you won't cause a change in
state.  Turning the temperature down (or even off) while the element is
"off" similarly won't cause a change in state.  The real problem here is
that electric stoves don't have indicator lights to tell you when the
element is on or off (although I have read about some people adding such
lights to their stoves.)

> In the case of dimmer switches on lights, it's less clear to me.  If
> they worked by using potentiometers, then they would have to
> dissipate as much as 37 watts in the case of a light bulb that is 150
> watts at full brightness-- that still seems like a lot for such a
> small thing. But they also seem too small and cheap to be
> transforming to a lower voltage. Is there an electrician out there
> who can explain how these things work?

Mechanical dimmer switches are most definitely potentiometers.  I
occasionally see them used in this capacity in childrens' science fair

As for where the power goes, you are right.  When the potentiometer is
set to the same resistance as the lightbulb (96 ohms for 150W at 120V),
the power dissipated is approximately 37W.  And the potentiometer
dissipates this power as heat.  If it's improperly installed, this may
even be enough heat to start a fire.

This is why potentiometers that are designed for high-power applications
(like the ones in my office's conference room) usually have large metal
heat-sinks attached - this dissipates the heat into the room so it won't
build-up inside the wall or inside the switch.

This is also why potentiometers are only rated for certain power levels.
A pot designed for a 100W (which will dissipate up to 25W) circuit
should not be used with a 150W lightbulb or a series of bulbs that adds
up to more than 100W.  (It's also why you should never wire a ceiling
fan to a pot that is designed for lightbulbs.)

Now, not all "dimmer" type controls use potentiometers.  Some (more
expensive) use multi-tap transformers (like those inside the switching
power-supply in your computer) or solid-state voltage regulators
instead.  They are will have different power-dissipation
characteristics.  You probably won't see these in used in any
residential application, however.

-- David

From: Stan Tenen <meru1@...>
Date: Tue, 05 Sep 2000 10:11:28 -0400
Subject: Potentiometer and Yom Tov

This is in response to comments by Rick Turkel in m-j#44.

An electric circuit can also appear to be open-ended.  Originally,
telegraph lines were single wires, with the return through the real
ground (the earth).  Also, static discharges returned to earth, just
about the same way that water spouting from an open sprinkler would
"return to earth".

Molecules of water are a fairly good "mechanical" analogy to electrons
in a wire.  You can count molecules, and you can count electrons, and
each moves in its own medium -- water in an open tube, and electrons in
what to them is the open lattice of a conductor (microwave energy
travels only on the surface of the tube also, which is why microwave
wave guides are hollow, just like water pipes. At microwave frequencies,
it's rare to count electrons, however. <smile>)

Electricity is like fire, in that both "glow" from ionized particles.
Electrons in a conductor are somewhat like the hot ions escaping from a
gas flame, if you look at them one by one.  But with electricity, this
is invisible, and it can be completely microscopic.  There can be a flow
of just one electron.  We can't detect fire as individual ions, without
technical help, and the situation is similar to our neglecting
micro-organisms that we can't see when we clean vegetables.

And there's another consideration with electricity.  The AC current we
use for home lighting doesn't really flow anywhere.  The electrons just
oscillate back and forth.  There's no net flow.  This would be like
putting a rubber membrane inside of a water pipe.  You could still push
surges back and forth through the pipe alternately from one side and
then the other, but there would be no net flow beyond the small amount
of water that would be contained in the bulging rubber membrane, at

There is no technical way to unambiguously distinguish electricity from
fire.  The distinction must be halachic and based on tradition, and
consistency with other teachings.  There is never going to be a
technical answer to this one, IMO.

With regard to Mike Gerver's posting in m-j#45:

Older electric appliances are not electronic, and they make use of
either switches, or potentiometers that dissipate energy in their
resistance.  Switching units control the temperature of a burner by
connecting and disconnecting whatever coils do the heating.  First
they're full on, then they're full off.  That's why it's okay to turn up
the heat of a burner when it's full on, but not okay to turn it down
then; and that's why it's okay to turn down a burner when it's full off
but not okay when it's on.  The "okay" direction does nothing but bias
the effect, and there's no additional spark of any kind (not even
microscopic).  So, the older switching burner controls can be adjusted,
even on Shabbos, as long as it's possible to tell when the burner is
actually on and when the burner is actually off.

This is also the principle behind old-style _electrical_ thermostats.
You can turn the heat up when the contacts are closed (but not down),
and you can turn the heat down when contacts are open (but not up),
without making any sort of spark, not even microscopic.  Whether you
make this adjustment depends on how you feel about making an adjustment
on Shabbos, which is another issue, but there need be no spark, and no
fire, if it's done properly.

More modern _electronic_ controls use potentiometers, and diodes or SCRs
or triacs.  There is no resistor that gets warm, and there is no spark
of any kind within a diode, an SCR, or a triac.  (They function like
smooth water valves or faucets.)

But there is a question about the use of resistive mechanical
potentiometers, which is what is almost universally used in
light-dimmers and oven controls.  Here, there is a contact that is being
drawn across a resistive surface, and in some cases, jumping from
winding to winding, albeit with constant connection, but nevertheless,
with the possibility of micro-sparks (if they were visible sparks, the
potentiometer would wear out almost immediately).  These microsparks
could be ruled to be aish, or they could be ruled not to be aish. This
is not a technical question; this is a question of halacha and

It's also possible to make potentiometers that do not spark at all.  One
can use a light source and a vane to shield it from a light detector,
and moving the vane makes no spark.  This is exactly the same as opening
or closing an opaque door on a lantern.  It's also possible to use a
magnetic field, which never sparks either, and this may be why some
poskim permit magnetic hotel door locks on Shabbos.

These days, it is definitely possible to control electrical and
electronic equipment without any spark whatsoever, and in a way that is
completely analogous to the control of water by a faucet or light by a
lampshade.  Whether or not there are guidelines for doing this in a way
that's acceptable on Shabbos is never going to be a technical
question. It can only be decided in reference to tradition, by qualified

Meru Foundation   http://www.meru.org   <meru1@...>


End of Volume 33 Issue 48