# Several thought experiments on the difference between direct and indirect measurements

I have read several books regarding basic concepts in physics but I'm still puzzled by own lack of knowledge. I shall not give definitions, because I presume we are all familiar with them. I do not have an answer to the following questions:

1. I want to know a length in inches. I've measured it with a line in meters and then converted the value to inches. Was this a direct or indirect measurement?
2. I want to know how many people are there in a given room. If I do the counting myself - that's a direct measurement. But I've chosen someone else do the counting and report it to me. What kind of measurement will that be?
3. Can every indirect measurement be converted to a direct one by constructing a machine that does all the steps and gives a one line answer?

3.2. I have a group of people that can measure and calculate everything. This group I can call a tool. If I make them measure something will that constitute as a direct measurement for me?

4. I want to make a measurement of a derived unit. Is it true that I can never do this with a direct measurement because first I need to measure the basic units and all other units in a chain of mathematical operations?

• This looks like a pure linguistics question to me. There is no official definition of what a 'direct' measurement is, as you must already know by refusing to give one. So you're just investigating what people would call edge cases, like asking whether a tomato is a fruit or a hotdog is a sandwich. The answer just about every physicist would give to all these question is, who cares? Apr 1, 2018 at 9:47

Measurement means comparison with a standard or etalon.

In your first example the etalon is a measuring tape or a ruler for example. This is a direct measurement because you compare a length with a length. And the measurement has a uncertainty because lengths on the macroscopic scale are measurable only to a measurement uncertainty, which depends from the used measurement instrument.

In your second example a direct measurement is to count the people in the room and this is a direct measurement independent of who and how may people carried out the measurement and who uses this measurement. An indirect measurement is it to weigh the people in a room (all together) and than conclude about the number of people using an average weight of say 80 kg.

Taking into account the above, it should not be necessary to answer the following questions.