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  1. tech99

    I Grating element of an acoustic grating

    I know the gratings work, and they were used for a large screen TV display in about 1939 in the form of the Jeffree Cell. However, it never occurred to me before that the grating effect must be pulsing on and off at acoustic frequency. With the Jeffree Cell I believe the cell used an acoustic...
  2. tech99

    This is why EM interference can be an issue...

    A CRT requires a line scan frequerency which is at about 16 kHz (625 system) and is delivered to the scan coils at around 25 Watts. Before the days when interfence from computers and broadband became so severe, line scan interference was very bad. It creates harmonics every 16 kHz right up the...
  3. tech99

    B Why can't two laser beams from two sources Interfere if they have a difference phase?

    Any two waves will interfere, but if they have different frequencies the interference pattern will be moving all the time, usualy too fast to see. It will repeat itself at the difference frequency. If, however, the frequencies are locked, then the inteference pattern will be stationary. If one...
  4. tech99

    Cheapest, easiest solar cell manufacturing methods on the Moon

    How will the Natural World of Earth, our precious biosphere, be saved by putting solar panels on the Moon?
  5. tech99

    Sea breeze and Chinese lantern question

    It might not be sea breeze as it could be the gradient wind. There is also variation of wind direction/speed with height.
  6. tech99

    Automotive Engine mechanisms during coasting (in drive mode) and engine braking

    I think it is a good idea for the drive train (and the driver) to keep the wheels connected to the engine when "coasting". In this way, the momentum of the rotating engine and flywheel wil make it less likely the road wheels cold stop turning or lock. In such a case, steering is impaired or lost.
  7. tech99

    Stargazing Possibility of detection of Earth approaching objects by gravity?

    The Arecibo radio telescope can obtain radar images of objects such as this which optical telesopes cannot see. In addition, the Sun is not a particularly strong noise source for radio waves.
  8. tech99

    I Can we connect two laser sources to make a stronger beam with a larger radius?

    Do we get speckles in the far field? Not sure.
  9. tech99

    I Can we connect two laser sources to make a stronger beam with a larger radius?

    If the lasers are side by side, we would see the "double slit" pattern superimposed on the spot. The two patterns multiply in terms of relative field strength. The problem is how to make a coaxial combiner with zero loss. Then the spot would be the same size as for one laser but double the...
  10. tech99

    I Can we connect two laser sources to make a stronger beam with a larger radius?

    It seems to me that if we place two lasers side by side, then at a great distance, greater than about diameter^2/(2*lambda), the beams will overlap. The spot will be the same size as for one laser, and the two beams, being incoherent, will add on a power basis, so the intensity of the spot will...
  11. tech99

    Accounting for mutual inductance in a Hartley Oscillator

    There are several ways of drawing a Hartley oscillator. The usual way I have seen is given like this and is a very convenient arrangement because no RFC is required, one side of the tuning capacitor is grounded (avoiding hand-capacitance) and no RFC is required. Of course, bias and decoupling...
  12. tech99

    I Strange approach to the line-fed slot antenna electromagnetic problem

    Sorry I can't do the maths, but I do know that the radiation from a slot is caused by the acceleration of charges in the metal. Wide slots and narrow slots radiate just the same. So the ground plane is the antenna.
  13. tech99

    I Why is the emissivity of metals low?

    I think Aluminium Oxide is a good dielectric and can support long waves efficiently but it has a crystalline structure which scatters waves with a wavelength comparable to the size of the crystals, such as light.
  14. tech99

    B Purely Capacitive AC Circuit -- Seeking intuition for why why voltage lags behind current

    A mechanical analogy is a spring. If we consider spring extension the analogue of charge, and force analogous to voltage, then extension is proportional to force, and Q is proportional to V. If you connect the spring to a rotating crank, so its length follows a sinusoidal pattern, then the force...
  15. tech99

    I Why is the emissivity of metals low?

    If the metal is polished, we can see that it creates an image of the source, which is a coherent secondary source and radiates almost as much power as it receives. If the surface is irregular, we see many secondary sources of radiation which are incoherent - they are of random phase. So the...
  16. tech99

    I Please Explain My Polarized Light Experiments

    It seems as if for this case the laptop is producing linearly polarised light and the camera is removing one component of it. I presume this rejection process is phase sensitive when the light passes through the mirror, which seems to be acting as a birefringent medium.This phase sensitivity...
  17. tech99

    Stopping an 80 Hz wave

    I appreciate the problem you are in. I hope that touching the unit will allow the cause to be found. However, these low frequencies tend to be carried by "conduction" through building walls etc and even the ground. The best approach, if the owner will agree, might be to mount the offending unit...
  18. tech99

    I Randomizing phases of harmonics

    The traditional view, which I learned as a young engineer in telecommunications where we were designing long telephone lines, is that the ear does not hear phase differences. In other words, the group delay/frequency response of a transmission system for audio is unimportant. Some telephone...
  19. tech99

    I Please Explain My Polarized Light Experiments

    Re the laptop observations, not absolutely sure but it sounds as if your laptop has linear polarisation. You were able to nul this out with the camera analyser used the right way round when the linear filter is in front. When you reverse the camera filter, the linear polarised light from the...
  20. tech99

    I Please Explain My Polarized Light Experiments

    Well to make a start on the many questions... My understanding of the camera is that it has a linear polariser first, which produces ther artistic efects, and then a circular polariser which then destroys the polarisation so as not to upset the camera electronics. With glass the polarisation...
  21. tech99

    How do my successive diffraction patterns look?

    I am a hands-on engineer so I am not sure I can be of much help. If the aperture radius is 1, I presume that means 1 wavelength. In this case 83 wavelengths for case 6 seems too great for near field conditions.
  22. tech99

    How do my successive diffraction patterns look?

    Is the sequence top left is far field and bottom right is near field? In which case near field is narrower. I cannot quite picture your radiating cylinder and which plane it is in. Can you say how far each diagram is from the aperture in terms of wavelengths, and what are the dimensions of the...
  23. tech99

    I Light speed and refractive index

    Regarding refractive materials for microwaves, I made a big prism, about 200mm side, from paraffin wax. This worked similarly to an optical one, including the demonstration of total internal reflection. In about 1895, Jagadish Chandra Bose used a thick copy of the Rail Timetable to demonstrate...
  24. tech99

    I Light speed and refractive index

    With a "normal" lens the light slows down on entering the glass and is bent towards the normal. But if the lens is made of something which allows light to speed up, then the light is bent the opposite way. In other words, away from the normal. So a convex lens becomes a diverging lens. In my...
  25. tech99

    I Light speed and refractive index

    Just for interest: I once tried to demonstrate that the sound of a bell inside a bell jar, which we evacuate with a pump, cannot be heard, whilst at the same time microwaves travel through the jar undisturbed. Unfortunately for me (as often happens), the microwaves decreased slightly with the...
  26. tech99

    Electrical conduction through air

    Yes, the oxide film on metals ij light contact is an insuator below about 1V and this is quite reliable. Even shaking does not break the film very easily. This is the basis of the coherer radio detector, which is biased to about 1V and utilises electron tunneling to create a threshold effect.
  27. tech99

    Electrical conduction through air

    As I mentioned before, I would be interested to know if you have conduction with voltages below about 1 volt.
  28. tech99

    Electrical conduction through air

    Thank you for an interesting reply. I think that 10VAC is sufficient to break down the oxide film. I have done experiments with loose contacts like this. At voltages of a fraction of a volt I think you will find there is no path. This is the same action as the radio detector called the coherer...
  29. tech99

    Max lifting capacity for an average human adult

    Do we have different limits for women?
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