The Copley Medal is the most prestigious award of the Royal Society, conferred "for sustained, outstanding achievements in any field of science". It alternates between the physical sciences or mathematics and the biological sciences. Given annually, the medal is the oldest Royal Society medal awarded and the oldest surviving scientific award in the world, having first been given in 1731 to Stephen Gray, for "his new Electrical Experiments: – as an encouragement to him for the readiness he has always shown in obliging the Society with his discoveries and improvements in this part of Natural Knowledge". The medal is made of silver-gilt and awarded with a £25,000 prize.