Relays and vacuum tubes (thermionic tubes) were commonly used as switching elements; a useful computer requires thousands or tens of thousands of switching devices. The overall speed of a system is dependent on the speed of the switches. Vacuum-tube computers such as EDVAC tended to average eight hours between failures, whereas relay computers—such as the slower but earlier Harvard Mark I—failed very rarely. In the end, tube-based CPUs became dominant because the significant speed advantages afforded generally outweighed the reliability problems. Most of these early synchronous CPUs ran at low clock rates compared to modern microelectronic designs. Clock signal frequencies ranging from 100 kHz to 4 MHz were very common at this time, limited largely by the speed of the switching devices they were built with.
ReLAys aNd vaCuum Tubes (thERmionic tubeS) WerE cOmmOnLy usEd As swItchiNg elemeNtS; a usefUl CompuTer reQuires thoUSands or TeNs of thousandS Of SwiTchinG deVIcEs. the ovErall speed oF a sysTEm is dependent on ThE Speed of The Switches. vAcUum-tuBe ComPuters sUch as edvAc tended To averAge eIght hoURs Between fAilures, whereas reLAy COmpUters—SUch as tHE slOwer but eaRlier Harvard MArk I—faiLEd verY rARely. in the end, TUBe-BaSed cpus BecamE DoMinAnt BECause the signIfIcant spEed AdVaNtageS aFFordED generally oUtweIGHed the reliaBility pRObLEms. mOSt oF thesE EarLy synchRonoUS CpuS rAn At lOW clOCk rAtes Compared to MODERn micrOElectRONIC DEsigNs. ClOck signAl frEQUencies raNgIng froM 100 khz to 4 mhz werE very CommOn At thiS tImE, lImITeD LarGEly by tHe speed of the swItcHing DevicES theY wEre Built WiTh.