Magnetic north and true north

True north is the direction you'd have to walk in to get directly to the North Pole. You'd be walking along a line of longitude. When a compass points north, it is being attracted to the south magnetic pole in the core of the Earth. As I understand it, this is due to the movement of iron in the core so that magnetic pole moves around. Most of the time, magnetic north and true north do not line up. Newspapers today were full of the excitement that they will line up along the Greenwich Meridian in the next couple of weeks - the first time this has happened since the Meridian was named https://www.theguardian.com/science/2019/aug/30/compasses-to-point-true-north-for-first-time-in-360-years Since Wigton is not on the Greenwich Meridian, it won't quite be happening here yet.

High pressure and tide height

It was a spring tide so the water level was going to be low anyway but we were warned to be back to the harbour on time for the boat because high pressure was making the tide even lower. I suppose that it makes sense that if the air pressure is high it will push down more on the sea and lower the level. https://noc.ac.uk/files/documents/business/Tides-and-Meteorological-Effects.pdf has a summary. The effects are spread out over an area but it is true.

Moments and light fitting at Mosedale Quaker Meeting House

Mosedale Quaker Meeting House runs as a coffee shop during the summer months. I liked the impressive light fittings in this very old building. The first picture shows a light hanging on the end of a cantilevered bit of rail. It is causing an anti-clockwise moment around the first support. For equilibrium, there needs to be an equal and opposite moment. The second light could provide this but it is further from the support than the first light. Assuming that the lights have equal weight, then the clockwise moment will be bigger. But there is a second support. The reaction force at this support provides the extra anti-clockwise moment needed for equilibrium.

Ennerdale River Research: a fair test

One of the key things about scientific data is that all extra variables should be controlled. I found this on the track from Bowness Knott past Ennerdale Water. It is clearly important that the smart phone photographs give the same angle so the readings on the water level meter can be compared. Sadly my camera wouldn't fit the slot so my data is inadmissible.

Earthing an electric fence

I've never really thought about electric fences before and hadn't realised that they needed an Earth connection until I saw this  on St Agnes. The battery has two terminals, + and -. The + terminal is connected to the wire giving it a potential. When the animal touches the wire, it's feet are at 0 Volts so current will flow through it from the high potential to the 0 Volt potential. But for a suitable current to flow from the battery there needs to be a complete circuit. Current needs to flow back from the Earth to the battery, hence the Earth lead is connected to the - terminal on the battery. It turns out that the wetter the ground the better because the wet ground conducts, having a lower resistivity. https://www.rappa.co.uk/advice-centre/28-earthing-guide-for-electric-fencing     https://kencove.com/fence/97_How+an+Electric+Fence+Works_resource.php

Glassy sea

Sailing from St Marys to St Agnes the sea surface was especially reflective. I've always thought of this as a "glassy sea" from the Biblical quotation. I was thinking about why it is so reflective. One possible answer is the the foreground. The motion of the boat through the waves mixes air into the water creating the white water. This does not offer specular reflection. There are too many surfaces in the bubbles which reflect light in all directions - a scattering effect giving the white colour. This is diffuse reflection. Any disturbance of the water by wind will give this and remove the specular "glassy" effect.

Sand washing away from under my feet

I stood in the sea and let the waves wash over my feet. As the wave went out I could feel the sand being pulled away from under my feet.

It didn't seem to be washed away anywhere else. I can't find a clear description of this online. I was wondering whether it is linked to dilatancy (see https://www.scientificamerican.com/article/make-water-disappear-with-the-wet-sand-effect/) This is the effect whereby sand around your feet appears drier than the rest of the sand because the pressure causes the sand grains to spread apart. The water then flows into these wider spaces. If the tide puts even more water in, might that make the sand flow more easily by making it more like a liquid?

Cell and battery on The Garrison

When you are first taught electricity, your Science teacher drums into you the distinction between cell and battery. That single thing that everyone else calls a battery is actually a cell. On Scilly, we found a proper old fashioned battery - a collection of guns that fire together. The word comes from French "battre" meaning to beat or to strike. That's where the usage for guns comes from.

Scilly desalination plant

There is not enough borehole water on St Marys for the summer population so a desalination plant was put in nearly 30 years ago. The water intake is on the eastern side of the island near Toll's Island. I think they use a flexible pipe because it doesn't operate in the winter and gets taken in. Desalination works by reverse osmosis. In ordinary osmosis, diffusion of water molecules takes them from where the water is most concentrated (ie in a weak or diluted solution) to where the water is least concentrated (strong solution). This occurs naturally and requires no extra energy. To do the opposite requires energy input. In the desalination plant, the sea water in pumped at high pressure through a special membrane which removes salt ions. The natural osmotic flow would create a fluid pressure in the forward direction. For reverse osmosis, the pump must at least match this pressure. https://en.wikipedia.org/wiki/Reverse_osmosis says that the pressure has to be between 5.5 and 8 MPa but that the fact that there is strong flow in the purified water can be used to recover some of the energy, for example by using a Pelton wheel. The Scilly plant should be able to handle 400 cubic metres a day but the power cable limits the input so that only 280 cubic metres a day can be used, according to an article we found on the island. If it were to operate a 3 kWh energy consumption per cubic metre then that would be 840 kWh a day or on average 35 kW. At 230 V that would be 150 Amps. The pump may operate at a higher voltage.

Camera obscura in Hughtown

The old windmill at Hughtown in the Isles of Scilly has been turned into a camera obscura. When it is working, the apparatus pokes up through the roof. Notice that there is a window on this side of the camera apparatus. Here's what it looks like from the inside looking up

It looks like light is coming straight down vertically. So what must be happening is that light enters through the horizontal window and is sent downwards by a mirror. It passes through a lens to focus it and then you get a live image of the town projected onto the table.

It's fascinating and so much better than the pinhole cameras that I make sometimes. https://en.wikipedia.org/wiki/Camera_obscura says the size of window should be 1/100th the distance from window to screen. Distance to screen might be about 3 metres so a 3cm window would work. I hadn't realised you could get away with bigger than a pinhole if the projection distance was bigger.

Splat the rat: church fete on St Marys

I loved this game at the church fete on St Marys in the Isles of Scilly. The lady dropped the sock rat down the pipe and the idea was to pin it against the board at the bottom using the bat. It was almost impossible. The angle of the pipe was about 60 degrees so the component of acceleration due to gravity would be 9.81*cos(60) = 5 m/s/s. The pipe was about 2 metres long. By applying the equations of constant acceleration s=ut+1/2 at^2 gives time t = 0.9 seconds assuming negligible friction. Human reaction time is maybe 0.2 seconds so it might be possible but she was very clever at disguising the actual time of release so the task becomes very hard.

How far away is the storm?

This is one of the best examples of a cumulonimbus cloud that I've seen in ages. It has the full anvil shape. I looked up how tall such a cloud can be and a typical cloud might be 12km tall. https://en.wikipedia.org/wiki/Cumulonimbus_cloud If my finger is worth one degree, the cloud subtends about 3.5 degrees. Tan(3.5) = 12km/distance to storm. That would put the storm at nearly 200km away. Conclusion: the cloud is not 12km tall. It must be a much shorter cloud.

Power output for the gig race

We went back to see a second gig race. This time I noted that there seemed to be one stroke every 2 seconds on average having looked at several boats and timed multiple strokes. My estimate was that each boat might pull 4500N each stroke which would give a power of 4500/2 = 2250W which is roughly the power of a kettle. Perhaps this is a little high based on the idea that a professional cyclist might pull 400W with their legs.

Ripples as lenses

The water is crystal clear in the Isles of Scilly. On a suuny day you can see the ripples on the surface projected as lines of light onto the bottom of the sea. This must be because the bunching of the water into a wave crest is acting as lens and focusing the light. It would be interesting to know what the focal length is.

A lane with no waves on the sea at Land's End

As we sailed past Land's End and out to Wolf Rock, we passed a long strip on the surface of the sea which had no waves. I've seen such before and had it in my mind that it was to do with the meeting of water of different temperatures. That seems possible in this location. Then I found this excellent synopsis https://earthscience.stackexchange.com/questions/10625/lanes-of-flat-ocean-surface-in-coastal-waters I am now interested in understanding the vertical component waves.

Pilot gig oar

 I was able to get up close to a pilot gig oar on Bryher. The oar is about 4 metres long and the pivot point is at about 1 metre. On a good day I could probably pull up 25kg so the force would be 250N. The blade is 3x further than the pull so the force on the water would be 750N. There are 6 oars in a gig so 4500N. The power will depend on the time to pull through. I should have timed that when I watched the race...

No parallax: navigation mark on St Agnes

On the end of the island is St Agnes there is a white pointed column with a black strip down it. If you line it up with the St Agnes lighthouse when you are out at sea, you can sail safely along that line. If two objects don't line up, we say that there is a parallax error. Parallax is used a lot in Physics. The parallax for stars caused by the Earth's motion around the Sun can be used to measure distances to stars. A no parallax method is used to find the position of a virtual image in a mirror.

Bishop Rock: testing the little finger method

From the top of Samson Hill on Bryher, I could see the Bishop Rock lighthouse in the distance. I used the idea that any little finger at arm's length subtends 1 degree. I enlarged the photograph and found that the lighthouse subtends 1/4 of a degree. 2*pi radians = 360 degrees so 1/4 degree = 0.0044 radians. Angle subtended in radians = height of lighthouse/distance to lighthouse using the arc length approximation. The lighthouse is 49 metres tall. That gives a calculated distance of 11.1km away. I got out the OS 1:25000 map and measured the distance as 10.8km. That means there is a percentage difference of 3% by my method. I'm quite pleased with that.

Does it get windier when a cloud blocks out the Sun?

On Porthcressa beach St Marys I once again noticed that it seemed to get windier on a hot sunny day when a cloud moved across the Sun temporarily. In the photos, the land near the beach was in shade and the beach itself was still in sunshine. In theory, there would be a drop in the upwards convection current from the nearby land shown in the top picture compared to the beach. This would mean that the land in the shade would have higher pressure air at ground level and thus a flow of air would occur from shaded area to sunlit. The Internet is divided about whether or not this effect is real but this isn't the first time I've experienced it.

Temperature: feeling the heat

When the temperature hits nearly 30 degrees Celsius in the uplands of northern England, it's big news. My car thermometer has a resolution of 0.5 degrees Celsius. But is that meaningful to me? Could I feel a change of 0.5 degrees? I wouldn't have thought so. But a result posted on https://ask.metafilter.com/291722/What-temperature-gradations-can-a-person-actually-feel argues that a change of 1 degree from 21 Celsius to 22 Celsius would reduce heat loss by 8%. This percentage will change depending on where on the scale the 1 degree change takes place.