As a chemical engineer,I can always try to explain

These errors are just because of the difference in the values of coefficient of thermal expansion.I had explained these things a couple of weeks ago to Bill

This is what I wrote:

Most substances expand when they are heated.Thermal expansion is a consequence of the change in average separation between constituent atoms of the object.(Assume atoms of an object to be connected to each other by stiff rings,for simplicity)In case of solids,this average separation is quite small (about 10

^{-10} to 10

^{-12} meters)As temperature increases,the atoms oscillate with greater amplitude and the separation between atoms increases,consequently the object expands.Reverse process occurs in case of cooling.

Linear Expansion: Suppose you have a rod of length 'L' at a temperature T Celsius,and its temperature changes from T to T1,then the change in length of the rod is given by L*Alpha(greek letter)*(T1-T),the Greek letter alpha is called

*coefficient of linear expansion*.The new length of the rod now will be L+ L*Alpha(greek letter)*(T1-T) Obviously,higher the temperature difference,higher is the expansion/contraction.

Now,because the linear dimensions of the rod change,it follows that its surface area and volume changes as well. The change in volume is given by V*Gamma(Greek letter)*Change in temperature. (V is the original volume). Gamma is called

*coefficient of volume expansion*.Alpha and Gamma are related as : Gamma=3*Alpha

Average values of alpha for some metals are- (per degree Celsius) (Multiply alpha by 3 to get value of gamma)

Steel-1.2*10

^{-5} Copper-1.7*10

^{-5}Brass-2*10

^{-5}Aluminum-2.4*10

^{-5}These values are extremely small,and the thermal expansion process might not be significant for small temperature changes.However,if you keep a bimetallic in oven at about 200-250C,you will find a change in its structure.

(These values and above expressions involve a lot of maths and experimentation)

Now in the case of coin Andrew has posted..The outer ring is nickel brass and inner ring is cupro nickel.From above values,the value of alpha for copper is less than that for brass.That means,for the same change in temperature,the outer ring will expand/contract more than the inner ring.Therefore,an allowance must be provided for expansion in case there's high temperature difference.(I think) the spikes are provided for the same reason.Note that it may not be necessary to provide such an allowance in a country like India where we don't have large temperature fluctuations like you have..

Any more doubts?

Aditya