Diffraction through net curtains

I noticed a street light diffracting through net curtains. It makes a cross shape because there are horizontal slits which diffract light vertically and vertical slits that diffract light horizontally. On an enlarged picture you can see that the spikes are subdivided. This shows the orders of diffraction. The colours were visible but haven't photographed well.

Thomas Young does something that isn't Physics in Worthing

Thomas Young is someone who ought to be a household name. I know him as the man who proved that light is a wave by demonstrating diffraction and interference effects. He also worked on the properties of solid materials, giving his name to the Young Modulus, a measure of how difficult a material is to stretch. But I didn't know that he also played a part in translating the Rosetta Stone. Apparently he figured out that the hieroglyphics representing translations of non-Egyptian names were phonetic. Hence the sounds of the hieroglyphs could be worked out. A recent biography described him as "the last man who knew everything". They were exciting times when it was possible to do world class work in many fields.

Components of magnetic field

 We did this experiment to measure components of magnetic flux density B. There was a.c. in the large copper coil which created a rapidly alternating magnetic flux. There was thus a large enough rate of change of flux density in the small yellow search coil to induce an alternating emf. We varied the angle of the search coil so that it was detecting different components of the flux. The largest flux is detected in the position shown below.

We displayed the induced emf on an oscilloscope and measured the peak-to-peak voltage, halving it to read the peak emf. This procedure reduces the percentage uncertainty in the measurement. By Faraday's Law EMF = rate of change of flux linkage = d Nphicos(theta)/dt = d(Nphi)/dt .cos(theta). We plotted peak emf against cos(theta). We got a reasonable straight line. The gradient will be the peak rate of change of flux linkage.

Smartphone compass on Ullscarf

My friend uses a Smartphone compass. I had a theory that it works by GPS comparing the position of the phone with known north. Turns out that this isn't true. The phones have magnetometers in them. These are often Hall probes which are doped semi-conductor. When this is placed in a magnetic field, the charges in the semi-conductor are displaced laterally relative to the current, creating a PD across the Hall probe. PD is proportional to magnetic flux density. I have seen suggestions that small arrays of Hall probes inside measure the flux density differently so allowing software to calculate the direction of greatest flux density - ie north.

Measuring up at Center Parcs

We found this at Center Parcs near Penrith. It was a display of the heights of sports stars. It was as bilingual as ever, giving metric metres and Imperial inches. I've been reading Leonard Susskind's book Quantum Mechanics: The Theoretical Minimum. He comments on why the Planck constant is such a stupidly small number by noting that it is really because humans are too big. The units that we use are based on the size of our bodies, as shown in the picture. The most fundamental scale is really down with the sub-atomic particles. We are just too big.

A Pale Line in the Sky from Scoat Fell

I climbed Scoat Fell on a beautiful day when the cloud mass was gathering in the north. You can make out the cloudy haze above High Stile. The top photograph is as normal and the bottom was taken through a polarising filter. There was a distinct pale level above the cloud. The sky does get paler blue as you get nearer to the horizon as Mie Scattering becomes important but I think this was more pronounced. My hypothesis is that above the cloud there was an increased level of water vapour which led to a more pronounced Mie Scattering effect. This is the scattering of all colours of light by water molecules - all colours so looking white.

Automatic Warning System: magnets on the railway

I spotted this from the railway bridge at Kennington near Oxford. We guessed that it was to do with signals and would be magnetic. I thought it would be a new thing but they have been around for decades. I realise that I have seen them before but that I've never seen a clean, new one. It is an Automatic Warning System AWS ramp. There is a permanent magnet in it which sets up the train's emergency brake system when the train rolls over. I can't find out whether the train detects a magnetic pulse in an electric circuit or whether the magnet works a mechanical switch. There is also an electromagnet which has the opposite polarity. If this is working, it deactivates the emergency brake system. There is human control over the system - the driver has to acknowledge the pulses. If the signal is a stop signal or similar warning, the current to the electromagnet is turned off which means that the emergency braking system is not deactivated. The really clever bit is that the system is set so that if there is a power cut, the emergency brakes will not be deactivated. It is fail-safe. So that's why one magnet is permanent and the other is an electromagnet. https://en.wikipedia.org/wiki/Automatic_Warning_System