Electrical activity going through the heart can be measured by external skin chest electrodes. The electrocardiogram (ECG) registers these activities from electrodes which have been attached onto different places on the body. In total, twelve leads are calculated using ten electrodes. They can be used for exercise test, operating room, and monitoring. This device is disposable, and without it, the doctors cannot observe the heart rate. It is produced in two sizes, for adults and for children.