This thermometer is Mercury free and completely safe to use. It uses alcohol and kerosene to measure temperature. The thermometer is mounted on a clear Mylar sheet backing to keep it on top of the eggs. A thermometer is essential for monitoring the temperature inside an incubator, which is imperative to the incubators process for chicken and other eggs. This thermometer features easy-to-read graduation marks that make keeping an eye on the temperature simple and straightforward. This egg incubator thermometer with backing measures 4.5 inches wide by 5.5 inches high.