a The game later became widely known under the name ‘diabolo’.
c An electrical kit, containing a battery.
d An Asian vegetable gum, similar to rubber. It was used in golf balls and for electrical insulation, and has interesting optical properties.
e The surviving draft ends at ‘continually’ but the final ‘changing’ is clearly implied.
f Devices for measuring areas by tracing around their edges.
g In Parts I and II of the paper Maxwell called his rotating elements ‘vortices’. In later parts he used the alternative term, ‘cells’. For simplicity here we call them cells from the outset.
h The ratio is the number of electrostatic units in one electromagnetic unit. The electromagnetic unit is bigger because it takes a bigger charge to generate a given force by electromagnetic action than by electrostatics. The ratio has the dimensions of a velocity because electrostatic force depends only on the charge but electromagnetic force depends additionally on its velocity.
i The amount of energy needed to raise the temperature of one mass unit of the gas by one degree.
j Entropy is a measure of the disorder in a system. According to the second law of thermodynamics, it always tends to increase. A small change of entropy is defined as the amount of heat transferred in a reversible process divided by the temperature at which the heat is transferred.
k The Poynting vector is much used in telecommunications work; it represents the energy flow per unit area of an electromagnetic wave.