The exactly opposite properties of the two kinds of electrification justify us in indicating them by opposite signs but the application of the positive sign to one rather than to the other kind must considered as a matter of arbitrary ( random choice ) convention ( agreement ) , just as it is a matter of convention in mathematical diagrams to reckon poitive distance towards the right hand . The exactly opposite properties of the two kinds of electrification justify us in 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 .