After the widespread introduction of the safety lamp, explosions continued because the early lamps were fragile and easily damaged. For example, the iron gauze on a Davy lamp only needed to lose one wire to become unsafe. The light was also very poor (compared to a naked flame) and there were continuous attempts to improve the basic design. The height of the cone of burning methane in a flame safety lamp can be used to estimate the concentration of the gas in the local atmosphere. It was not until the 1890s that safe and reliable electric lamps became available in collieries.