Books like Capital, Vol. 2: The Process of Circulation of Capital

So far we have considered part of the working day when the worker simply offsets the value that the capitalist pays him, as an constant magnitude, it is invariable reality in production conditions. Beyond this necessary time, work could be extended by two, three, four, five, six, etc. hours. According to the greatness of this extension, the rate of surplus value and the length of the day varied. If the necessary labor-time was constant, the whole day was variable otherwise.The shortening of the necessary labor as well correspond to the extension of the surplus labor, or some of the time that the worker consumes far in reality for himself, would turn into working time for the capitalist. The limits of the day would not be changed, but its work division necessary and surplus labor.Moreover, the duration of surplus labor is fixed as soon as data are the limits of the day and the daily value of labor power. If it is five shillings - Gold sum which incorporated ten hours of work when the worker has to work ten hours a day to compensate for the value of its force paid daily by the capitalist, or to produce an equivalent keeps it needs for its daily maintenance. The value of these daily subsistence determines the value of its strength [1], and the value thereof determines the daily duration of his necessary work.