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that the potential difference in volts is proportional to the concentration 

 of the hydrogen peroxide in millimoles per liter. 



Analog computers at best can do a limited number of types of opera- 

 tions. However, they are rapid, comparatively inexpensive, and have 

 a precision comparable to most biophysical data (that is, at best ± 0. 1 

 per cent, but more usually ±10 per cent). When similar types of 

 problems are solved repeatedly, analog computers may be the most 

 economical method of obtaining numerical answers. Many analog 

 computers, and all digital computers, use electronic techniques. In 

 this chapter, two electronic analog computers are described to illustrate 

 this general approach. Both use electrical potentials to represent 

 different physical variables. 



Digital computers are far more versatile than analog computers. 

 Their versatility arises from their ability to make choices. The machine 

 senses when it has finished one operation ; then it automatically goes on 

 to the next. A digital computer can compare numbers, and, depending 

 on their relative sizes, can go to one of several alternative next steps. 

 A high speed digital computer can complete in 1 second as many as 

 10 5 elementary operations, such as addition and multiplication of 10 

 digit numbers. Digital computers are very rapid, very sensitive, and 

 very expensive. They can be programmed, that is, instructed, to do 

 any of an almost infinite variety of different problems, provided the 

 programmer is sufficiently ingenious. Digital computers are discussed 

 in the last section of this chapter. 



3. A Bone-Density Analog Computer 



Analogs can be formed for many different types of problems. The one 

 to be considered here uses several analogies. This computer was 

 designed for the quantitative measurement of bone densities from X-ray 

 films. It is impossible to determine the bone density or bone calcium 

 directly from an X-ray photograph of a bone in a limb because one 

 knows neither the conditions of exposure nor the sensitivity curve of the 

 film as used and developed. To determine the last two, a wedge of an 

 aluminum-zinc alloy is exposed with the bone. Suitably made wedges 

 have the same shape of absorption and scattering curves when plotted as 

 a function of wavelength as does the calcium phosphate in mammalian 

 bones. For each point along the image of a bone it is possible to find a 

 wedge thickness that gives the same film darkening. These equivalent 

 thicknesses could be integrated numerically to find the equivalent wedge 

 mass of a given slice of bone. If the volume were known from other 



