1mA = __A

Hint : The milliampere is a unit used to measure electric current. To solve this question, we need to draw the analogue of comparison of the size of a millimeter to that of a meter.

Complete step by step answer
The Amp, or Ampere is the SI unit of current. The International Organization for Standardization symbol for Ampere is $ A $ .
The sign $ {\text{mA}} $ refers to milliampere or milliamp. This is a smaller unit of the SI unit of current, Ampere.
This prefix forms a very handy way of describing decimal fractions of a unit as well as decimal multiples. I expect you know for example the length of $ {\text{1 meter}} $ is $ {\text{39}}{\text{.37 inches}} $ . A kilometer is a thousand meters. In the other direction, there is the centimeter ( $ {1 \mathord{\left/
 {\vphantom {1 {100}}} \right.} {100}} $ ) millimeter ( $ {1 \mathord{\left/
 {\vphantom {1 {1000}}} \right.} {1000}} $ ), micrometer ( $ {1 \mathord{\left/
 {\vphantom {1 {1000000}}} \right.} {1000000}} $ ) and so on.
These prefixes are used for a number of units. So now, the relative size of a milliamp can easily be compared with an amp.
The value of $ {\text{1mA}} $ is thus smaller than the value of $ 1A $ and it is used to calculate smaller values.
 $ {\text{1mA = }}{{\text{1}} \mathord{\left/
 {\vphantom {{\text{1}} {{\text{1000}}}}} \right.} {{\text{1000}}}}{\text{A}} $ .

Note
Sometimes we are dealing with very small currents, like $ {\text{0}}{\text{.002 A}} $ . It is much more convenient to use a smaller unit. There are $ {\text{1000 mA}} $ in one A. So we can write the same current from our example as $ {\text{2 mA}} $ .
The "milli" in front of the "amps" is a prefix indicating the value is a thousandth. One milliamp is one-thousandth of an amp. One millivolt would be one-thousandth of a volt, and one milliohm would be one-thousandth of an ohm in resistance, and so on. Going the other way, one has kilo, mega, and giga, (kiloamp, Mega-amp, & Giga-amp, respectively) meaning thousand, million, and billion, respectively.