Place your hand on the left side of your chest below your left nipple you will most likely feel your heartbeat.
That gentle pumping motion it is what is keeping you alive by maintaining circulation of blood throughout your systems. it has in fact kept you alive from the day you were born (actually long before that) and has never stopped. Some would argue that it is the most important organ in the human body.
Do you know why it never stops?
Well the heart is kept beating by its inherent electrogenic capacity and a complex interplay between various hormonal and nervous factors.
This electrogenic capacity is the ability of the heart to generate its own electrical current, and transmit it throughout itself. Some of this electrical current is dissipated throughout the body and can be picked up using very sensitive electrodes that are placed at strategic points on the body surface. This is the principle behind electrocardiography
ECGs are an integral part of modern medicine, to such an extent that most of medicine but especially cardiology will be completely lost without it. They are very cheap and non invasive, and can give a lot of information.
For example this is a normal heart. You can tell the rhythm, heart rate, the direction and angle to which the heart is deviated, and so on.
This is a person with likely mitral valve stenosis, causing the left atrium to enlarge might not be life threatening but still a cause for concern
This is the heart of a person with pericardial effusion. A potentially life threatening condition in which the heart is squeezed and not allowed to expand and pump properly.
ECGs were invented more than a century ago and have continued to be used to save lives and improve health since then, even though only slight improvement have been made from the original invention. At least not until now.