2. The cup of melted wax and the climbing fuel

The lesson, step by step

1. Hook question

   SayAn oil lamp has a bowl to hold the oil. A candle has no bowl at all. So how does solid wax get up into the flame?

   Let your child guess before you read.

2. Read together

   Read pages 22 to 31, down to the line about the candle turned upside down.

   You see, then, in the first instance, that a beautiful cup is formed.

3. Watch the cup form

   Light the plain candle on its plate in a still room. After a few minutes a little pool of melted wax forms around the wick, with a firm rim. Look at it from the side. The rising warm air cools the outside, so the rim stays solid while the middle melts.

   Faraday says the pool stays level because of gravity:

   for the same force of gravity which holds worlds together holds this fluid in a horizontal position

   Safety: an adult lights the flame and stays right there; tie back long hair and roll up sleeves; set the candle on a plate on a fireproof surface, away from paper and curtains; keep a glass of water nearby.

4. Why fancy candles drip

   If air comes from one side, the cup is lopsided and wax runs over: that is "guttering." Faraday draws a lesson from it:

   It is not the best looking thing, but the best acting thing, which is the most advantageous to us.

   And another one, worth copying into the notebook:

   I hope you will always remember that whenever a result happens, especially if it be new, you should say, "What is the cause? Why does it occur?" and you will in the course of time find out the reason.

5. Climbing water (capillary action)

   "Capillary attraction!" you say, — "the attraction of hairs." Well, never mind the name: it was given in old times, before we had a good understanding of what the real power was.

   Try it two ways. (1) Pour a little colored water on the saucer next to the sugar cube and watch the color creep up the cube, just like Faraday's blue liquid creeping up his column of salt. (2) Fill one glass with colored water, set an empty glass beside it, and hang a twisted strip of paper towel from one into the other. Check it in an hour. This is Faraday's story of the towel left over the side of the basin that emptied the water onto the floor.

   SayThe wick works the same way. The melted wax climbs the cotton, the way the water climbs the paper.

6. Why the flame stays at the top

   Faraday explains that the melted fuel itself puts the flame out lower down:

   Now, the only reason why the candle does not burn all down the side of the wick is, that the melted tallow extinguishes the flame.

   Ask: What would happen if you turned a burning candle upside down? (The melted fuel would run onto the wick and put it out. Don't try it.)

7. Say it back and check

   Your child explains, in their own words, the path of the wax: solid, melted in the cup, up the wick, into the flame.

   Check questions:

   1. Why does the rim of the cup stay solid?

   2. What is capillary attraction? Give an example from your house.

   3. Why are the fluted candles "bad burning" candles?

8. What we know now

   Today we call it capillary action. It happens because the liquid is attracted to the fibers of the wick (or paper) more than it holds itself together, so it creeps up through the tiny gaps.