Capital, Volume One: The Process of Production of Capital by Karl Marx

That portion of the working day which merely produces an equivalent for the value paid by the capitalist for his labour-power, has, up to this point, been treated by us as a constant magnitude, and such in fact it is, under given conditions of production and at a given stage in the economic development of society. Beyond this, his necessary labour-time, the labourer, we saw, could continue to work for 2, 3, 4, 6, &c., hours. The rate of surplus-value and the length of the working day depended on the magnitude of this prolongation. Though the necessary labour-time was constant, we saw, on the other hand, that the total working day was variable. Now suppose we have a working day whose length, and whose apportionment between necessary labour and surplus labour, are given. Let the whole line a c, a–b–c represent, for example, a working day of 12 hours; the portion of a b 10 hours of necessary labour, and the portion b c 2 hours of surplus labour. How now can the production of surplus-value be increased, i.e., how can the surplus labour be prolonged, without, or independently of, any prolongation of a c?

Although the length of a c is given, b c appears to be capable of prolongation, if not by extension beyond its end c, which is also the end of the working day a c, yet, at all events, by pushing back its starting-point b in the direction of a. Assume that b'–b in the line ab'bc is equal to half of b c a–––b'–b––c or to one hour's labour-time. If now, in a c, the working day of 12 hours, we move the point b to b', b c becomes b' c; the surplus labour increases by one half, from 2 hours to 3 hours, although the working day remains as before at 12 hours. This extension of the surplus labour-time from b c to b' c, from 2 hours to 3 hours, is, however, evidently impossible, without a simultaneous contraction of the necessary labour-time from a b into a b', from 10 hours to 9 hours. The prolongation of the surplus labour would correspond to a shortening of the necessary labour; or a portion of the labour-time previously consumed, in reality, for the labourer's own benefit, would be converted into labour-time for the benefit of the capitalist. There would be an alteration, not in the length of the working day, but in its division into necessary labour-time and surplus labour- time.