Page "Electric charge" Paragraph 31

It is the established convention of the scientific community to define the vitreous electrification as positive, and the resinous electrification as negative.

The exactly opposite properties of the two kinds of electrification justify our indicating them by opposite signs, but the application of the positive sign to one rather than to the other kind must be considered as a matter of arbitrary convention, just as it is a matter of convention in mathematical diagram to reckon positive distances towards the right hand.