is insulated by the cooking surface, and voltages generated in the pan are far too low to represent a shock hazard. The cooktop can detect whether cookware is present by monitoring power delivered. As with other electric ceramic cooking surfaces, a maximum pan size may be specified by the manufacturer, and a minimum size is also stated. The control system shuts down the element if a pot is not present or not large enough. If a pan boils dry it can get extremely hot – a thermostat in the surface will turn off the power if it senses overheating to prevent cooker failures and potential fires. Cooker surface Induction cooker tops are generally a low-thermal expansion glass-ceramic. The surface of the cooker is heated only by the pot and so does not usually reach a high temperature. The thermal conductivity of glass ceramics is poor so the heat does not spread far. Induction cookers are easy to clean because the cooking surface is flat and smooth and does not usually get hot enough to make spilled food burn and stick. The surface is brittle and can be damaged by sufficient impact although they must meet specified impact standards. Aluminum foil can melt onto the top and cause permanent damage or cracking of the top. Surfaces can be scratched by sliding pans across the cooking surface. Noise Noise is generated by an internal cooling fan. Also, audible electromagnetically induced acoustic noise (a high-pitched hum or buzz) may be produced, especially at high power, if the cookware has loose parts or if the layers of the pot are not well bonded to each other; cookware with welded-in cladding layers and solid riveting is less likely to produce this type of noise. Some users are more capable of hearing (or more sensitive to) this high-frequency sound. Other considerations Some cooking techniques available when cooking over a flame are not applicable. Persons with implanted cardiac pacemakers or other electronic medical implants are usually instructed to avoid sources of magnetic fields; the medical literature seems to suggest that proximity to induction cooking surfaces is safe, but individuals with such implants should check with their cardiologists. Radio receivers near the induction-cooking unit may pick up some electromagnetic interference. Because the cooktop is shallow compared to a gas-fired or electrical coil cooking surface, wheelchair access can be improved; the user's legs can fit below the counter height