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Thanks again. Some extra head scratching... Being the distance from the parabola to the directrix equal than from the parabola to the focus, the first post line 'L' (concave side) does not qualify as directrix as I said in post #4. P1-Q1 is not equal to P1-F. But the total paths are the same lenght !.

Thanks. OK, then a directrix can be at the convex side or at the concave side as "L" in the graph; not only as typical tutorials with the directrix shown at the convex side. That was the point. To find in a parabolic antenna if wavefronts would interfere or even cancel with an uneven parabola surface and how 'unsmooth' are the limits for a given wavelenght. I guess that a half wavelength of the intended received signal as 'bump' or 'valley' in the parabola surface would contribute to signal cancellation. Is it right ? Edited: Am receiving 12GHz with an 'ironed' now-unobtanium parabola after tree limbs fall damaged some surfaces. 12GHz ---> ~ 2.5cm ----> half wavelength is 1.25 cm. Keeping surfaces as flat as possible under 1.25cm bumps and valleys is the aim, correct ? ----> http://s588.photobucket.com/user/Innernet/media/HFuWave.jpg.html?sort=3&o=92

Hi. By its definition, the distance from a parabola to its focus and directrix are the same. Well, this is not a directrix as I knew; it is at the opposite side : Question is -- are the lengths of the paths Q1 - P1 - F and Q2 - P2 - F and Q3 - P3 - F The same ?

All that information provided would bring worthless calculations without knowing wheel/tire diameter.

Never had coffee in my life. Do not know how they work. Suspect are very simple, but never played with one nor its guts. Any coffee maker tinkerers here that can suggest which type/model could be used to distill some alcohol, as a kitchen countertop appliance ? A dimmer to bring temperature down ? A mash holding section that can be cleaned ? Eliminating/adding few parts ? A quart per month will suffice. There is tricked water distillers/purifiers on the market that can distill ethanol, but the point is if a coffee maker can. Am not interested in legalese. All in the name of science !

Diagnose, diagnose, diagnose. Get one of these, they are cheap enough, and been around for decades : ----> http://www.ebay.com/sch/items/?_nkw=computer+post+card&_sacat=&_ex_kw=&_mPrRngCbx=1&_udlo=&_udhi=&_sop=12&_fpos=&_fspt=1&_sadis=&LH_CAds=&rmvSB=true Or, remove the hard drive, set aside unmolested and put another proven good hard drive (used, old or new) in the PC. Better if with Linux operative system (free) If failure persists or not, you discern being the hard drive/operative system, or the PC mainsomething/supply

There is no other kind of errors in accidents. Only human errors.

Device arrived to my mailbox. Instructions say to position near ceiling for 'natural' gas and near floor for LPG, whatever that be -liquified petroleum gas or liquified propane gas-. Installed. Done.

Cannot tell for smoke; but gas detectors use the Davy mesh pointed by John : And if the pendant detector is battery powered and kept charged by AC, fine. But if powered by AC alone, is not that 'cool' to need the light on for it to work. studiot : The leak being from an appliance or other seems not determined at the article ----> http://www.dailymail.co.uk/news/article-2142801/Cheltenham-house-gas-explosion-Elderly-woman-Betty-Hodgkiss-miraculously-escapes.html Not much remaining to perform an examination of cause. Perhaps a newer report gives clues.

You were assuming it was radiation powered, now being battery operated. Battery ? Probably not much; that may be why is AC powered... A little less significant as the reason houses have mandatory smoke detectors ? Only one house needed it among hundreds around.

Thank you, studiot. The americium pellet sensors are for smoke detection, as far as I knew. I do not think radioactivity is used to perform the detection for combustible gases. Combustible gases use other sensing methods as infrared and catalytic, no radiation involved. Some sensing modules look like : ----> https://duckduckgo.com/?q=combustible+gas+detector+sensor+module&t=canonical&iax=1&ia=images A light description of a vendor product ----> http://smart-prototyping.com/MQ-9-carbon-monoxide,-combustible-gas-sensor-detection-alarm-module.html (There is also a .pdf data sheet in there) And similar to the one I ordered ----> http://www.tech-faq.com/combustible-gas-detectors.html

Radiation ? On a gas related subject ? --> Radon. Danger for leaks is Kaboom ! I do not think on how they work; am after an optimal location

"high frequency" means nothing. What is the frequency you want the piezoelectric element to vibrate ? Will you apply alternating voltage to it or will you apply alternating motion to it ?

Perhaps one of these days the paranoia will expand to radon, to fit your question... meanwhile, will stick to combustion gas.

Thanks. The sensor ordered and to arrive soon, is : ----> http://www.ebay.com/itm/Plug-In-Combustible-Gas-Detector-Alarm-Sensor-Voice-Warning-and-Battery-Backup-/311164368273?hash=item4872d77591 There is other CO and smoke detectors already installed, but my paranoia is for the gas itself. Will later modify to install/extend a buzzer and leds indicators to the upper level and the sensor itself in the basement, where all pipes are.

For the first time am moving to a house with gas service. Am afraid of it. All pipes have been checked for leaks by a professional. Still afraid, am installing a gas detector alarm. Where is it better to install; near the ceiling or near the floor ? I believe it is natural gas; whatever that be... methane, propane, butane, hexane...

Hi. A space probe/craft traveling at very high speed, in need to slow-down or land on a remote destination, has to waste its energy or is there a method that can recover some of it and store it for later takeoff or whatever energy consuming actions needed ?

Hi all. Could the experiences, academic learning, general knowledge, memories be copied/transferred from and old fart to a youngster, by electrical¿? tapping, transplant of some brain tissue, replication or other ? Has it ever been tried in animals; is there enough knowledge about brain-memory to attempt such, or would have to be delayed for a guessy another century ? Please move to speculations if deserved.

How does air gets in the hollow of bird bones ? Is it somehow renewed or stale ?

Do any of you in U.S. have kids on high school in order to find out how they are currently taught about forces/thrust ? Sorry, I have to leave now to the store to get three pounds of sixteen-penny nails.

Hi all. As a kid, learned forces being the unit lbf/Kgf as poundsforce/kilogramsforce. Later at university, the newton showed up. What do current physics teachers/text books teach in USA to youngsters in school ? Newtons or not ? Are lbf/Kgf abolished ? Should they be for a correct start ?

Thanks. Correct, but a speaker is what was requested from me as diver and involved on acoustics and electronics. Plus my curiosity on testing such. For simple attention getter, I have used these very successfully for a long time : ----> http://www.scienceforums.net/uploads/monthly_01_2013/post-295-0-21064300-1358975176.png I believe the speaker cone immersed in fluid should propagate sound bidirectionally well enough; that is why I asked here. Some valid opinions are contrary and I respect them, but will proceed with the experiment anyway, as it is extremely simple.

Hi acme. Details omitted, the purpose is to send unidirectional voice commands from a boat P.A. to a crew of divers to hook a rope that tows which selected item to another nearby location, and release, repeatedly; the speaker being by the end of the rope, and they do not want to purchase commercial equipment for only an hour of use. Am currently putting together the contraption with the speaker inside an oil filled intravenous bag (it's sturdier), no bubbles, heat sealed, cable trough its hose exit, sealed, all inside a flatty plastic box perforated both sides. Will let know success or failure. Yes, could be done from the surface too with a floating proper speaker.

A nice way to let time pass when you wait for your order... ----> Certainly opens a multitude of opportunities.

Had those a while ago. Doctor explained that 'floaters' (threadlike strands, which can be knobby and semi-transparent) suspended in the vitreous gel move with sudden eye movements. When they contact/brush the retina receptors in their motion, their 'abrasion' produce the darting flashes sensation. Hope it helps.