Defining the kilogram

For over two centuries the kilogram has been defined as the mass of a platinum cylinder kept in Paris, making it the only SI unit still defined in terms of a specific object. But this definition is to be replaced

Susan Street

Kilogram 18, the UK’s official copy of the IPK

National Physical Laboratory © Crown Copyright/SPL

The terms in bold link to topics in the AQA, Edexcel, OCR, WJEC and CCEA A-level specifications, as well as the IB, Pre-U and SQA exam specifications.

This article shows how complex units can be broken down into combinations of SI base units, before going on to outline the role of electromagnetic force in measurements that underlie a new definition of the kilogram.