An instrument for measuring heartbeats

Answer 1

A stethoscope can be used to listen to the heart, the auscultations and valve sounds.

There are many ways to "measure" a heart beat. The most effective tool is the ECG (electrocardiogram) which measures electrical activity of the heart. Because the electrical activity (electrical signals originating from the sino-atrial and atrio-ventricular nodes of the heart) is a direct indicator of the physical activity of the heart (contraction, relaxation, valve activity, etc) you can determine almost everything you need to know about the heart using an ECG, including its physical orientation within the mediastinum (sac containing the heart within the chest cavity).

The time between each electrical impulse can be measured, thus giving elapsed time between each beat, and allowing you to determine how many times the heart contracts (beats) every minute - known as heart rate (HR). Presumably, this is what you mean by "measuring heartbeats", but an ECG is capable, as explained, of doing much more.

Hope this helps, if you would like me to expand or go into more complex detail, let me know.

Source: I am a Medical Student in the UK.