I want to make variable current circuit 1to 100ma?

Question

It is for digital ammeter calibration

Answer 1

Making a calibration source is far more difficult than you believe, especially if the ammeters are used for anything other than personal use.

Most organizations (insurance, banking, licensing) require traceability to NIST (National Institute of Standards and Technology) for legal protection. Those costs alone preclude making your own calibration device.

Look into renting or purchasing a used one (already calibrated) from and equipment rental firm such as ElectroRent, TestEquity, MetricTest, etc.