Your horse should be haltered and adequately restrained. To take your horse's temperature with a mercury thermometer, start by shaking the mercury line down below the 98 degrees Fahrenheit marker. The following works well for right handed people.

Lubricate the thermometer with lubricant or saliva. Make contact with the horse's left shoulder and move down the side of the horse. Firmly grasp the tail with your left hand and lift it up. It may help to prop your elbow on the horse's hip. With a twisting motion, gently insert the thermometer 3/4 of the length in. Secure the alligator clip to hairs of the base of the tail. Wait between 1.5 to 2 minutes before removing and reading.

My preference is still a mercury thermometer but many prefer digital. Mercury thermometers have become difficult to find and are being phased out over time. A broken mercury thermometer is an environmental hazard. Please also see the skill on taking temperature with a digital thermometer.

Do not try to insert a mercury thermometer in a resistant horse. You may break the thermometer.

Most horses can be easily taught to accept a thermometer. Teach your horse to allow this skill, so you can perform it easily when you really need to.

For resistant horses, you must first desensitize the area. If a horse will not allow you to raise and handle its tail and touch the skin around the anus, it will not allow you to insert the thermometer. It is best to teach your horse to accept this procedure when they are healthy so that they do not resist when you really need to get it done.

To perform this skill, you will need a glass mercury thermometers that has a tiny loop at the end. Tie a 12 inch string to the loop with an alligator clip at the other end. After inserting the thermometer, clip the alligator clip to the hairs at the base of the tail.

Should the thermometer start to fall out, the alligator clip and string prevent it from falling and breaking or getting lost. The alligator clip is also a good reminder that the thermometer is in place, should you momentarily forget that you are taking your horse's temperature during a stressful moment.

Do not forget to remove the thermometer!

You can lubricate the thermometer with spit. However a more proper approach is to use a lubricant, such as K-Y Jelly. A twisting motion often makes it easier to insert, but do not ever force it.