Put half the raspberries in a preserving pan and place over a gentle heat.
Crush the raspberries with a potato masher and simmer for a couple minutes.
Tip in the remaining raspberries, leave whole, and stir.
Add the jam sugar and stir occasionally until dissolved. Check the sugar has completely dissolved before increasing the heat to medium.
Bring to a gentle rolling boil and cook for 5 minutes before testing for setting point. Setting point is reached when the jam thermometer reaches 104℃ or 220F.
Or use the wrinkle test - remove your pan from the heat and drop a little jam onto an ice cold saucer. Then gently push the jam with your fingertip - if the jam crinkles, setting point has been reached. If not, boil the jam for a couple of minutes more and check setting point again.
Once setting point is reached, use a ladle to transfer your jam into warm, sterilised jars and then seal.
Store in a cool, dry place out of the sunlight and use within 12 months.