A Water Spiral

The hose is attached to a speaker. Speakers work by vibrating at certain frequencies, pumping the air at those frequencies, which is picked up by our ears and interpreted by our brains as sound. Video cameras work by taking pictures at a very rapid rate, usually 24 or 30 frames per second. Our eyes send signals to the brain at a rate of about 14 frames per second, so they interpret the much faster video frame rate as continuous motion.

In the video above, the camera is taking 24 frames per second. The speaker is vibrating at the same rate, 24 pulsations per second. The speaker drives the hose to vibrate at that same rate, making a little circle 24 times per second. Every time the camera takes a picture the hose is back to the same place, so the camera cannot detect the motion of the hose, even though it's actually making a circle.

That's why the water appears motionless. Imagine that instead of a continuous stream of water, it's actually made up of droplets, oil hose shot out of the hose 240 times per second. That means every time the hose makes a little circle, it shoots out 10 drops. They come out in order, one after another, each at a slightly different angle around the circle from the one before it. If you connect the droplets you'd get a spiral pattern. The water itself is moving straight away from the hose, but the spiral pattern is what we see as we mentally connect the dots.

I love how the video maker changes the vibration rate of the speaker a bit to change the water's apparent motion. At 25 vibrations per second the water appears to slowly wind around, because each droplet from the hose is caught at a different position due to the unmatched camera and vibration rates. Then he switches it to 23 Hz and it looks like the water is moving up, backwards, into the hose! Very cool, fuel hose and totally an illusion.