Every horse person should have several thermometers. Thermometers are available in mercury column (traditional type) and digital. Both types work well. The advantage of the mercury column type is that the glass tube has a ring to which can be attached string and alligator clip. This enables the thermometer to be clipped to the tail. It’s shape is also easier to insert and keep in the rectum.
The advantage of digital thermometers is speed of measurement and wide availability. Mercury thermometers are becoming harder to find because of regulation on mercury while digital thermometers are easy to find at any drug store.
Be sure to shake down column-type thermometers. The thermometer should be inserted at least several inches through the anus. Use a twisting action and plenty of lubrication for easy insertion. Be consistent in depth of insertion, as different depths may yield inconsistent measurements. If the tip of the thermometer is against a fecal ball, it may artificially lower the temperature.