### ORIGIN OF DX

DX is an early telephone term for distant exchange. "This is correct. In the 1960s I worked in many telephone exchanges around the United Kingdom. On the old-fashioned switchboards with plugs and cords, circuits coming in from distant exchanges had a label marked "DX" above the jack socket. The operator would plug into the circuit and announce the name of her exchange, as confirmation to the distant operator that she was through to the correct destination. 73 de G3NYY"

It is also defined in Funk & Wagnall's as Distance.

The term DX appears in some math formulas as distance of x.

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However, Phil, K7PEH Physics and Math reports. This may be true in a few very limited areas where the author has penned their own unique definition to dx. However, by far the majority of instances that dx is used in mathematics is to refer to the "derivative of x". As you might know, the derivative of a function is the rate of change of that function with respect to something else. When dx is used in math, it never stands by itself but must also refer to the "other something else" that is changing in respect to x. For example, the following formula: dx = f'(x,y)dy Defines dx as the rate of change of x in the function of x and y with respect to the change in y. I have included the notation as f'(x,y) which is commonly read as "f prime" to indicate the derivative of the function. Mathematically this is not necessary but it is the traditional definition of the differential dx/dy. Since x often is used to specify a coordinate in space, the term dx is often used to represent an infinitesimally small change in that coordinate of x. In this sense you might say that dx is a measure of distance but this is a very limited definition and not the general definition of dx in mathematics. But, even in this limited sense, the term dx is never read as "distance of x", it is always read as "derivative of x".

WebMaster: Note So much for the urban legend of DX is the distance of X -- but makes a good story.

**Saailin**

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