Why does the Time Traveler make the psychologist press the lever?

The psychologist seemed most prone to believing the Time Traveller and the experiment to be a fake. Thus, the Traveller chose him to pull the lever in order to prove it was not trickery. 

The Very Young Man stood behind the Psychologist. We were all on the alert. It appears incredible to me that any kind of trick, however subtly conceived and however adroitly done, could have been played upon us under these conditions.

The Time Traveller looked at us, and then at the mechanism. `Well?' said the Psychologist.

`This little affair,' said the Time Traveller, resting his elbows upon the table and pressing his hands together above the apparatus, `is only a model. It is my plan for a machine to travel through time. You will notice that it looks singularly askew, and that there is an odd twinkling appearance about this bar, as though it was in some way unreal.' He pointed to the part with his finger. `Also, here is one little white lever, and here is another.'

The Medical Man got up out of his chair and peered into the thing. `It's beautifully made,' he said.

`It took two years to make,' retorted the Time Traveller. Then, when we had all imitated the action of the Medical Man, he said: `Now I want you clearly to understand that this lever, being pressed over, sends the machine gliding into the future, and this other reverses the motion. This saddle represents the seat of a time traveller. Presently I am going to press the lever, and off the machine will go. It will vanish, pass into future Time, and disappear. Have a good look at the thing. Look at the table too, and satisfy yourselves there is no trickery. I don't want to waste this model, and then be told I'm a quack.'

There was a minute's pause perhaps. The Psychologist seemed about to speak to me, but changed his mind. Then the Time Traveller put forth his finger towards the lever. `No,' he said suddenly. `Lend me your hand.' And turning to the Psychologist, he took that individual's hand in his own and told him to put out his forefinger. So that it was the Psychologist himself who sent forth the model Time Machine on its interminable voyage. We all saw the lever turn. I am absolutely certain there was no trickery. There was a breath of wind, and the lamp flame jumped. One of the candles on the mantel was blown out, and the little machine suddenly swung round, became indistinct, was seen as a ghost for a second perhaps, as an eddy of faintly glittering brass and ivory; and it was gone--vanished! Save for the lamp the table was bare.

Every one was silent for a minute. Then Filby said he was damned.

The Psychologist recovered from his stupor, and suddenly looked under the table. At that the Time Traveller laughed cheerfully. `Well?' he said, with a reminiscence of the Psychologist. Then, getting up, he went to the tobacco jar on the mantel, and with his back to us began to fill his pipe.