Joanne Martino, age 11, of Staten Island, N.Y., for her question:

WHAT IS AN ELECTROCARDIOGRAPH?

An instrument used to diagnose disorders of the heart is called an electrocardiograph. The equipment detects and records the electrical impulses that normally develop in the heart and spread through its muscle with each beat. The electrocardiograph evolved from a string galvanometer invented by Willem Einthoven in 1903.

A recording made by an electrocardiograph is called an electrocardiogram. It is often abbreviated as EKG. Impulses from a normal heart make records of a specific size and shape. When abnormal conditions are recorded, changes in the pattern are shown.

Impulses from the heart are gathered at four points on the body to make an EKG. Electrodes, which are wires from the machine, are attached on each of the patient's wrists, on his left leg and on his chest.