A transmutation beam was a device used by the Federation in Project Atlas. It converted matter into biomagnetic recordings and stored them in crystals or tapes.
In 2267, the Federation used giant transmutation beams to convert the endangered population of Styra into biomagnetic recordings, which were stored in a handheld sphere. Another, smaller transmutation beam was used on the USS Enterprise to restore a schnauzer, on which the technology had been tested. After the Styran recordings were stolen by Allura and Anzar, Spock used the transmutation beam to convert Captain James T. Kirk and a security team into biomagnetic recordings, then trick Anzar into restoring them. (TOS comic: "The Hijacked Planet")