Marx expounds the question in the following way:

Let us assume that half the surplus value of Department I is being accumulated. The capitalists, then, use 500 for their consumption but augment their capital by another 500. In order to become active, this additional capital of 500 must be divided, as we now know, into constant and variable capital. Assuming the ratio of 4 to 1 remains what it was for the original capital, the capitalists of Department I will divide their additional capital of 500 thus: they will buy new means of production for 400 and new labour for 100. This does not present any difficulties, since we know that Department I has already produced a surplus of 500 means of production. Yet the corresponding enlargement of the variable capital by 100 units of money is not enough, since the new additional labour power must also find adequate consumer goods which can only be supplied by Department II. Now the circulation between the two large departments is shifting. Formerly, under conditions of simple reproduction, Department I acquired 1,000 consumer goods for its own workers, and now it must find another 100 for its new workers. Department I therefore engages in enlarged reproduction as follows:

4,400c+1,100v.

Department II, in turn, after selling these consumer goods to the value of 100, is now in a position to acquire additional means of production to the same amount from Department I. And in fact, Department I still has precisely one hundred of its surplus product left over which now find their way into Department II, enabling the latter to expand its own reproduction as well. Yet here, too, the additional means of production alone are not much use; to make them operate, additional labour power is needed. Assuming again that the previous composition of capital has been maintained, with a ratio of 2 to 1 as regards constant and variable capital, additional labour to the tune of 50 is required to work the additional 100 means of production. This additional labour, however, needs additional consumer goods to the amount of its wages, which are in fact supplied by Department II itself. This department must therefore produce, in addition to the 100 additional consumer goods for the new workers of Department I and the goods for the consumption of its own workers, a further amount of consumer goods to the tune of 50 as part of its aggregate product. Department II therefore starts on enlarged reproduction at a rate of 1,600c + 800v.

Now the aggregate product of Department I (6,000) has been absorbed completely. 5,500 were necessary for renewing the old and used-up means of production in both departments, and the remaining 500 for the expansion of production: 400 in Department I and 100 in Department II. As regards the aggregate product of Department II (3,000), 1,900 have been used for the increased labour force in the two departments, and the 1,100 consumer goods which remain serve the capitalists for their personal consumption, the consumption of their surplus value. 500 are consumed in Department I, and 600 in Department II where, out of a surplus value of 700, only 150 had been capitalised (100 being expended on means of production and 50 on wages).

Enlarged reproduction can now proceed on its course. If we maintain our rate of exploitation at 100 per cent, as in the case of the original capital, the next period will give the following results:

I.4,400c+1,100v+1,100s=6,600
II.1,600c+800v+800s=3,200
Total:9,800

The aggregate product of society has grown from 9,000 to 9,800, the surplus value of Department I from 1,000 to 1,100, and of Department II from 750 to 800. The object of the capitalist expansion of production, the increased production of surplus value, has been gained. At the same time, the material composition of the aggregate social product again shows a surplus of 600 as regards the means of production (6,600) over and above those which are actually needed (4,400 + 1,600), and also a deficit in consumer goods as against the sum total made up by the wages previously paid (1,100v + 800v) and the surplus value that has been created (1,100s + 800s). And thus we again have the material possibility as well as the necessity to use part of the surplus value, not for consumption by the capitalist class, but for a new expansion of production.

The second enlargement of production, and increased production of surplus value, thus follows from the first as a matter of course and with mathematical precision. The accumulation of capital, once it has started, automatically leads farther and farther beyond itself. The circle has become a spiral which winds itself higher and higher as if compelled by a natural law in the guise of mathematical terms. Assuming that in the following years there is always capitalisation of half the surplus value, while the composition of the capital and the rate of exploitation remain unchanged, the reproduction of capital will result in the following progression:

2nd year:I.4,840c+1,210v+1,210s=7,260
II.1,760c+880v+880s=3,520
Total:10,780
3rd year:I.5,324c+1,331v+1,331s=7,986
II.1,936c+968v+968s=3,872
Total:11,858
4th year:I.5,856c+1,464v+1,464s=8,784
II.2,129c+1,065v+1,065s=4,259
Total:13,043
5th year:I.6,442c+1,610v+1,610s=9,662
II.2,342c+1,172v+1,172s=4,686
Total:14,348