238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



growth mechanism of crystals, and innumerable other frontier regions 

 of knowledge will find the electron microscope indispensable. 



Even broader in its applications than the electron microscope is the 

 electronic computing machine. It not only supplies quantitative 

 answers to problems in fields governed by known laws, however in- 

 volved their application may be; it also greatly accelerates the dis- 

 covery of laws governing physical phenomena through the rapid 

 comparison of the predictions of a hypothesis with actual 

 measurements. 



There are two types of electronic computing machines : the analogue 

 computer and the digital computing machine. In the former the vari- 

 ables in the equation to be solved are represented by electrical quan- 

 tities, such as current or voltage, and the operations occurring in it, 

 by the characteristics of the circuit and tube elements making up the 

 computer. Both in principle and in function, the electronic analogue 

 computer is closely related to the mechanical analogue calculating 

 device, which, in the form of Bush's differential analyzer, ushered in 

 the era of large-scale computing machines. 



Examples of electronic analogue computers are MacNee's differential 

 analyzer for the solution of ordinary differential equations (1)^ ; Gold- 

 berg and Brown's simultaneous equation solver (2) ; and the numerous 

 auxiliary circuits employed, in radar technique and elsewhere, to 

 simplify the presentation of data. By its very nature the applica- 

 tion of the analogue computer tends to be specialized, its accuracy 

 limited.^ Even so, it has proved its usefulness in countless ways, in 

 a saving of time and effort and the elimination of human error. 



By contrast, the digital computing machines are quite universal in 

 their application and their accuracy is only limited by the number of 

 significant figures that can be entered on them. As the name implies, 

 the digital computer deals with numbers represented by a sequence of 

 digits. The primary difference between the electronic digital com- 

 puter and the familiar mechanical computing machine is a difference 

 in the speed with which the electronic machine can perform the ele- 

 mentary arithmetical operations. This quantitative difference, how- 

 ever, leads to important qualitative differences : if the time in which 

 the elementary arithmetical operations are performed is measured in 

 microseconds, the basic advantage of the high speed of the machine is 

 lost unless the time of transfer from one operation to the next also 

 is measured in microseconds. This necessarily eliminates the role of 

 a human operator from all intermediate operations and even pro- 

 hibits the employment of mechanical devices in the major portion of 



' Numbers In parentheses refer to references at the end of this paper. 



• Eventually, as In the simultaneous equation solver, a rapidly converging process of 

 iteration may be employed to achieve results of arbitrary accuracy. 



