Why "constant"?
Marx calls the constant part of the capital outlay "constant" because according to his labour theory of value, constant capital inputs - once produced, purchased, withdrawn from the market and used to create new products - do not by themselves add new value to output, or increase in value in the production process. Instead, the value of equipment and materials being used in production is conserved and transferred to the new product by living labor.
It is true that the ruling market prices for constant capital inputs could change after they have been bought for use in production, but normally this cannot affect those inputs (having been withdrawn from the market for use in production), only the market valuation of the outputs created from those inputs.
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