apparatus for effecting reactions , said apparatus comprising a plurality of reaction vessels for holding reagents , an electrically conducting polymer which emits heat when an electric current is passed through it , and control means for controlling supply of current to the polymer , the polymer being connectable to an electrical supply via the control means . the control means may be arranged such that different currents and therefore different temperatures can be achieved in each reaction vessel . certain novel reaction vessels are described and claimed . the apparatus are reaction vessels may be used in carrying out reactions which require multiple temperature stages such as amplification reactions such as the polymerase chain reaction .