Galileo Thermometer

Galileo Thermometers make fine gifts.  Particularly, the kind of fine gift you give when you can’t think of anything else to give.  You often see them in offices and kitchens used as decorations more than anything.  But they do indeed tell the temperature as their colored bulbs of glass float up and down the vertical glass tube.  How do these interesting objects work and why are they named after Galileo Galilei?  I thought I’d investigate.

Prior to early 1600’s, it was not known how to measure temperature.  Measuring temperature is one of those things we take for granted today.  Imagine not having any way to quantify hot and cold.  You might know that a certain thing happened when it was warm enough (water boiling, snow melting, or seeds sprouting, for example) but without a way to measure temperature predicting these events would be impossible.

In the early 1600’s, Galileo built a device called a thermoscope that detected changes in temperature.  He would partially fill a glass tube sealed at one end with water and invert it in a bowl of water.  As the temperature changed, the water in the tube would rise.  This device was affected by barometric pressure so it couldn’t be used to quantify temperature.  Others would do that with the invention of the thermometer.  Galileo’s contribution to the development of the thermometer was that he discovered that a sealed glass bulb suspended in a liquid will rise or sink as the temperature changes.  He hypothesized that the liquid’s density changed as temperature changed, causing the sealed bulb to sink or float.

The Galileo Thermometer works on this principal.  It consists of a sealed glass tube filled with water and a number of glass bulbs which are partially filled with colored aqueous alcohol.  When the bulbs are made, their densities are matched by adding varying amounts of liquid to each before sealing.  A small brass tag with a temperature stamped on it is attached to each bulb.  These are actually counterweights, each weighing slightly more than the next.  The counterweighted bulbs are put in the glass tube in the order of the temperatures stamped on the tags from lowest to highest.  My Galileo Thermometer has ten bulbs tagged from 64 degrees to 82 degrees in increments of two. 

Now, for some examples:  If the room temperature is colder than the lowest number, all the bulbs will float to the top of the tube.  If the room temperature is warmer than the highest number, all the bulbs will sink to the bottom.  If the ambient temperature is somewhere in between, the bulbs with numbers greater than the room temperature will float to the top while bulbs with numbers lower than the room temperature will sink to the bottom.  Thus, the room temperature can be read between the lowest floating bulb and the highest sunk bulb.

So the next time you see one of these thermometers gracing someone’s TV room, have a little respect.



Question of the Day: In the video above, is the ambient temperature warming or cooling?