10 Electrostatics Physical Principles [OH. i 



with a second substance, according as the first substance stands above or 

 below the second substance on the list. The following is a list of this kind, 

 which includes some of the most important substances : 



Cat's skin, Glass, Ivory, Silk, Rock crystal, The Hand, Wood, Sulphur, 

 Flannel, Cotton, Shellac, Caoutchouc, Resins, Guttapercha, Metals, Guncotton. 



A substance is said to be electropositive or electronegative to a second 

 substance according as it stands above or below it on a list of this kind. 

 Thus of any pair of substances one is always electropositive to the other, the 

 other being electronegative to the first. Two substances, although chemically 

 the same, must be regarded as distinct for the purposes of a list such as the 

 above, if their physical conditions are different ; for instance, it is found that 

 a hot body must be placed lower on the list than a cold body of the same 

 chemical composition. 



IV. Attraction and Repulsion of Electric Charges. 



12. A small ball of pith, or some similarly light substance, coated with 

 gold-leaf and suspended by an insulating thread, forms a convenient instru- 

 ment for investigating the forces, if any, which are brought into play by the 

 presence of electric charges. Let us electrify a pith ball of this kind positively 

 and suspend it from a fixed point. We shall find that when we bring a 

 second small body charged with positive electricity near to this first body 

 the two bodies tend to repel one another, whereas if we bring a negatively 

 charged body near to it, the two bodies tend to attract one another. From 

 this and similar experiments it is found that two small bodies charged with 

 electricity of the same sign repel one another, and that two small bodies 

 charged with electricity of different signs attract one another. 



This law can be well illustrated by tying together a few light silk threads 

 by their ends, so that they form a tassel, and allowing the threads to hang 

 vertically. If we now stroke the threads with the hand, or brush them with 

 a brush of any kind, the threads all become positively electrified, and there- 

 fore repel one another. They consequently no longer hang vertically but 

 spread themselves out into a cone. A similar phenomenon can often be 

 noticed on brushing the hair in dry weather. The hairs become positively 

 electrified and so tend to stand out from the head. 



13. On shaking up a mixture of powdered red lead and yellow sulphur, 

 the particles of red lead will become positively electrified, and those of the 

 sulphur will become negatively electrified, as the result of the friction which 

 has occurred between the two sets of particles in the shaking. If some of 

 this powder is now dusted on to a positively electrified body, the particles of 

 sulphur will be attracted and those of red lead repelled. The red lead will 

 therefore fall off, or be easily removed by a breath of air, while the sulphur 



