Although early computers such as EDSAC made successful use of mercury delay line memory, the technology had several drawbacks; it was heavy, it was expensive, and it did not allow data to be accessed randomly. In addition, because data was stored as a sequence of acoustic waves propagated through a mercury column, the device's temperature had to be very carefully controlled, as the velocity of sound through a medium varies with its temperature. Williams had seen an experiment at Bell Labs demonstrating the effectiveness of cathode ray tubes (CRT) as an alternative to the delay line for removing ground echoes from radar signals. While working at the TRE, shortly before he joined the University of Manchester in December 1946, he and Tom Kilburn had developed a form of electronic memory known as the Williams tube or Williams–Kilburn tube, based on a standard CRT, the first random-access digital storage device. The Baby was designed to show that the system was a practical storage device, by testing that data held within it could be read and written at the speed necessary for use in a computer.