One of my less popular videos involves using a wattmeter to measure the efficiency of an immersion heater. In response to someone's request, I recently wrote it out. Immediately below is the letter I sent to this instructor and further below is the written procedure. I am posting a copy on the vimeo website:

I did this lab for the first time last week with students by just talking them through it and writing things on the board. However, since I have to write it up sooner or later I just wrote it up now (attached as both a work and pdf file). Be sure to share the video with your students too so they can see how it is done.

A couple of things you may need to know: I did not do it yet with a hotplate because I did not have access to one last week, but I think it would be a relatively safe and foolproof addition to the experiment. I also increased the electricity applied from 20 to 40 watt hours because it gives more accurate results (unless you have a watt meter that is more sensitive). Speaking of foolproof, you may even leave out the step involving the immersion heater because they are hard to obtain and are easily abused and may even be a fire hazard: Last week I destroyed my immersion heater when I though I was plugging in something else and it was not in water. I knew something was amiss after smelling a buring odor, which was my immersion heater burning out!

If you think this is not enough to do, I suggest you modify the procedure to look at the efficiency of applying different amounts of watt hours (20, 40, and 60 watt hours). Good luck.

Items needed: a plug-in watt meter, a Celsius thermometer, a hot plate, a 1 liter Pyrex glass beaker, a watch glass that covers the beaker, an electric tea kettle that holes at least 1 liter, an immersion heater, and a thermos that holds at least 500 mL.


1) Place 500 mL of water and the thermometer into the 1-liter beaker.

2) Plug your watt meter into the wall then plug the hot plate into the watt meter with the heat turned off.

3) Set the watt meter to read kilowatt hours, making sure you are starting at “zero” (reset if necessary).

4) Record the initial temperature then turn on the hot plate to the maximum setting.

5) Read the watt meter and turn off the hot plate as soon as the watt meter reads 0.04 kilowatt hours (this is equivalent to 40 watt hours).

6) Record the final temperature and do the following calculations: (assume 1 mL water = 1 gram)

a) (grams of water) × (4.2 J / g °C) × (final temperature – initial temperature) = Joules of heat absorbed

example: (500 g water) x (4.2 J / g x °C) x (70°C - 20°C) = 105,000 Joules heat absorbed

b) (Watt hours consumed) × (3600 J / watt hour) = Joules electricity consumed

example: (50 Wh) × (3600 J / watt hour) = 180,000 Joules electricity consumed

c) (Joules heat absorbed) ÷ (Joules electricity consumed) = efficiency ratio

example: (105,000 Joules) ÷ (180,000 Joules) = 0.58 = 58% efficiency

7) Empty the beaker, let it the hotplate cool to room temperature, then repeat steps 1-6 this time covering the beaker with the watch glass.

8) Repeat steps 1-6 this time using the electric tea kettle with the top open.
9) Repeat steps 1-6 this time using the electric tea kettle with the top closed.
10) Repeat steps 1-6 this time using the immersion heater with the top open.
11) Repeat steps 1-6 this time using the immersion heater with the top closed.

Caution: An immersion must always be immersed in water when it is turned on or it will be permanently damaged. Furthermore, an immersion heater does not have an “on/off” switch; You can only turn it off by unplugging it. Never plug it in unless it is immersed in water!